The Place of Recurrent Novae among the Symbiotic Stars

ASPConferenceSeries,Vol.**VOLUME**,**YEAROFPUBLICATION****NAMESOFEDITORS**

ThePlaceofRecurrentNovaeamongtheSymbioticStars

J.Miko󰁗lajewska

N.CopernicusAstronomicalCenter,Bartycka18,00-716Warsaw,Poland

Abstract.Theobservationalpropertiesofrecurrentnovaeindicatethattheycanbedividedintotwosubclasses:systemswithadwarfandaredgiantsec-ondary,respectively.Thesecondtype–whichincludesRSOph–bearsmanysimilaritiestosymbioticstars.

1.Introduction

Symbioticstarsareinteractingbinariesinwhichanevolvedcoolgiant–eitheranormalMgiantinS-typesoraMiravariableembeddedinanopticallythickdustshellinD-types–transfersmaterialtoahotwhitedwarf.InsomecasestheMgiantisreplacedbyaG-Kgiant–thesearecalledyellowsymbioticstars,andthewhitedwarfisreplacedbyaneutronstars–thesearealsoclassifiedaslowmassX-raybinaries(e.g.V2116Oph/GX1+4).Thepresenceofanevolvedgiantisessentialtoformasymbioticbinary,andsotheremustbeenoughspaceinthesystemtoaccomodatesuchabigstar.Thus,symbioticstarshavethelongestorbitalperiodsandthelargestcomponentseparationsamongtheinteractingbinaries,andtheyareaveryattractivelaboratoryforstudyingvariousaspectsofinteractionsandstellarevolutioninbinarysystems(e.g.Corradi,Miko󰁗lajewska&Mahoney2003).

Thesubclassofrecurrentnovaewithgiantsecondaries–RSOph,TCrB,V30SgrandV745Sco–havethesamecompositionasthesymbioticbinaries,andsotheysharemanyphysicalcharacteristicswiththesesystems.Theaimsofthispaperaretopresentthestate-of-the-artinunderstandingofsymbioticbinaries,andtodiscusstheplaceofthesesymbioticrecurrentnovae(SyRNe)amongthesesystems.Inadditiontotheorbitalandstellarparameters,themechanismsofmasslossandaccretionaswellasthelinkbetweentheSyRNeandtheZAnd-typesystemswillbediscussed.2.

Orbitalparameters

Thedistributionsoftheorbitalperiods,massratiosandthestellarcomponentmassesforsymbioticstarshavebeenrecentlydiscussedbyMiko󰁗lajewska(2007).Sincethen,orbitalperiodhavebeenfoundforanother5galacticsystems,andspectroscopicorbitshavebeenderivedfor3systems(Fekeletal.2007;Gro-madzkietal.2007)whichincreasedthenumberofsystemswithknownorbitalperiodsandspectroscopicorbitsderivedfromthecoolgiantabsorptionfeaturesto70and32objects,respectively.TheupdateddistributionsareshowninFig.1,

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1000

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0.40.60.811.2

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Figure1.Orbitalparametersofsymbioticstars.Theshadedregionsde-notedifferentpopulations:theMCs,yellow,andsymbioticrecurrentnovae,respectively.

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andtheydonotaffectthepreviousconclusions.Inparticular,thereisnosys-tematicdifferencebetweenthesymbioticsystemsinthegalacticdiscandthoseinthebuldge,whereastheyellowsymbioticstars(lowmetallicity,halosystemswithaKgiantdonor)maysplitintotwopopulations,withPorb∼200–600days,and≥900days,respectively.ThesymbioticsystemsintheMagellanicCloudshavePorb≥900dayswhichisconsistentwiththelongerperiodtailofthegalacticS-typesystems.Thecoolgiantsmassespeakaround6M⊙,andthewhitedwarfmasses–around0.5M⊙,respectively.MoredetaileddiscussionoftheorbitalparametersisincludedinMiko󰁗lajewska(2003;2007).

Theorbitalparametersareavailableonlyfor2SyRNe:TCrBandRSOph(e.g.Belczynski&Miko󰁗lajewska1998;Brandietal.2008).Bothhaveorbitalperiods–227and453days,respectively–consistentwiththeshorterperiodtailofgalacticS-typesystems.Thereare,however,significantdifferencesbetweentheSyRNeandtheothersymbioticstars.InbothRSOphandTCrB,thecoolgiantisthelessmassivecomponent,withmass,Mg∼0.6–0.8M⊙,lowerthanthatofanyothersymbioticgiant,whereastheirwhitedwarfsarethemostmassive,withmasses,Mg∼1.1–1.4M⊙,sufficientforthemtobecomeIasupernovae.3.

Thehotcomponentanditsactivity

ThesymbiotichotcomponentshavebeenrecentlydiscussedbyMiko󰁗lajewska(2003,2007).Accordingtotheiractivityallsymbioticstarscanbesplitintotwosubclasses:ordinaryorclassicalsymbioticstarsandsymbioticnovae.

Thevastmajorityofthesymbioticsystemsbelongstothefirsttype,includ-ingbothnon-eruptivesystemslikeRWHyaandSYMus,andsystemswithZAnd-typeactivity.Theirquiescenthotcomponentsappeartoberelativelyhot

5(>∼10K)andluminous(∼1000L⊙)whitedwarfspoweredbymoreorlesssta-blethermonuclear(TNR)burningoftheaccretedhydrogen.ZAndandmanyotherclassicalsymbioticstarsshowoccasional1–3magoptical/UVeruptionsontimescalesfrommonthstoafewyears,whenthehotcomponentmaintainsaroughlyconstantluminosity,whereasitseffectivetemperaturevariesfrom∼105to∼104K.ThisactivitycanbebestexplainedbythepresenceofanunstableaccretiondiscaroundtheTNR-shellburningwhitedwarf(Miko󰁗lajewska2003;Sokoloskietal.2006).Theseclassicalsymbioticwhitedwarfsclusteraroundthemass-luminosityrelationsforstarsleavingtheAGBwithaCOcoreandthoseleavingtheRGBwithadegenerateHecoreforthefirsttime(seeFig.5ofMiko󰁗lajewska2003)whichsuggeststhattheycouldstillbehotattheonsetofmasstransferfromtheredgiantandsymbioticactivity.

Thesymbioticnovaearethermonuclearnovaeinasymbioticbinarysystem.Thereareeightknownsymbioticnovae,occuringinbothS-type(AGPeg,RTSer,V1329CygandPUVul)andD-typesystems(RRTel,V1016Cyg,HMSgeandRXPup).Theiroutburstsdevelopveryslowly:therisetomaximumtakesmonthstoyears,andthedeclinetothepre-outburststagecanlastdecades(seeFig.6ofMiko󰁗lajewska2003).Therecord-holderamongthemisAGPeg:itshotcomponentmaintainedaconstantluminosity,∼3000L⊙foratleast100years(Kenyonetal.1993).TheevolutionofRXPupwasmuchfaster:theconstant-luminosity(plateau)phaselastedforonly11years,andthemaximum

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plateauluminosity,∼15000L⊙,wasabout5timeshigherthanthatofAGPeg(Miko󰁗lajewskaetal.1999).Thesedifferencesinoutburstevolutionreflectdifferentwhitedwarfmasses:higherinRXPupthanthatinAGPeg.RXPupisalsoapossiblerecurrentnova(Miko󰁗lajewskaetal.1999).TheSyRNearecloselyrelatedtothesesymbioticnovae.ThemaindifferencebetweenthetwogroupsisthepresenceoftheverymassivewhitedwarfsintheSyRNewhichaccountsforbothrecurrenceandveryshorttimescalesoftheirnovaoutbursts.

BetweentheTNRnovaoutbursts,thehotcomponentsoftheSyRNe(in-cludingRXPup)showintrinsicvariabilityresemblingthehighandlowstagesoftheaccretion-poweredsystemsofCHCygandV694Mon(MWC560)aswellassomeoftheclassical(ZAnd)symbioticactivity,inparticular,theactivityofallthesesystemsischaracterizedbyverysimilartimescales(Anupama&Mikoa-jewska1999;Miko󰁗lajewska2003,andref.therein;Gromadzki,Miko󰁗lajewska&Lachowicz2008).BothintheSyRNeandCHCygavariableB/A/F-typeshellsourcewithLUV/opt∼10–500L⊙appearsduringthebright(high)state.HiBalmerandHeiemissionlinesarealsopresentbutHeiiandotherhigh-ionizationlinesarerarelyobserved.TheemissionlinefluxesrequirearatherhotsourcewithT>∼50000KandtheEUVluminosity,LEUV∼LUV/opt.Simi-lar,double-temperaturestructureisfoundinthemoreluminous,activeclassicalsymbioticstars.InAXPer,ARPav,FNSgr,andpossiblyothersystemstheshellabsorptionlinestracetheorbitofthehotcomponentandaremostlikelyformedinageometricallyandopticallythickaccretiondiscandgasstream(Quirogaetal.2002).InRSOphandTCrB,thehotcomponentbrighteningisassociatedwiththeappearanceofrapidphotometricvariability–flickering,sim-ilartothatobservedincataclysmicvariables(Anupama&Miko󰁗lajewska1999;Gromadzki,Miko󰁗lajewska&Lachowicz2008).ThesamecorrelationbetweenflickeringandactivitywasfoundinCHCygandV694Mon,andevenintheclassicalsymbioticsystemZAndduringitslastseriesoferuptions(Sokoloski&Bildstein1999).AlltheseapparentsimilaritiessuggestthatboththemultipleoutburstactivityofZAnd-typesymbioticstarsandthehighandlowstatesoftheSyRNeareduetounstabledisc-accretionontothewhitedwarf,withtheonlydifferencethatthewhitedwarfsintheformerburnstheaccretedhydrogenmoreorlessstablybuttheydonotinthelatter.4.

Thecoolgiant

Thespectraltypesofthesymbioticgiantsarenowrelativelywellestimatedbasedonred/nearinfraredobservations.Thecomparisonbetweensymbioticandsingleredgiantsinthesolarneighborhoodrevealsastrongbiastowardslaterspectraltypesintheformer–thespectraltypedistributionpeaksatM5forS-types,andatM6/M7forsymbioticMiras,andthatthefrequencyofMiravariablesishigheramongsymbioticgiants(Muerset&Schmid1999).ThesymbioticMirashavealsosystematicallylongerpulsationperiodsthansinglegalacticMiras,andtheyaresurroundedbyopticallythickdustshells(Whitelock2003).Thispredominanceofverylate,andthusmoreevolvedgiantsinsymbioticbinariessuggeststhatlargeradiusandhighmasslossisessentialfortriggeringsymbioticbehaviourinbinaries.Indeedsymbioticgiantstendtohavehighermasslossrates–∼10−7and∼10−6–10−5M⊙yr−1inS-andD-types,respectively–

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ascomparedtoaverageredgiantsandMiras(e.g.Miko󰁗lajewska2003,andreferencestherein).ThecoolcomponentsoftheSyRNefollowatleastsomeofthesetrends.Theirspectraltypes–M2inRSOph,M4inTCrB,andM5–6inV30SgrandV745Sco(Anupama&Miko󰁗lajewska1999),fallintotherangecoveredbytheS-typesystemswhereastheshortreccurencetimefortheirnovaeruptionsrequiresahighaccretionrate,∼10−7M⊙yr−1andconsequentlyahighmasslossratefromthegiant.

Althoughthesymbioticgiantsarepersistentlyclassifiedasnormallumi-nosityclassIIIgiants,theirhighmasslossratessuggestthattheymightbemoreevolvedthanaveragefieldgiantswithsimilarspectraltypes.Recently,Gromadzkietal.(2007)haveshownthatmostS-typesymbioticscontainlow-amplitudeSRbvariablesinsteadofnormal(nonvariable)giantswhichcanac-countfortheirhighmasslossrates.

Unfortunately,thereisonlyonesymbioticgiant,CHCygwithadirectlymeasuredradius.TheIOTAinterferometricobservationsgaveRg=300R⊙in1996(Dycketal.1998),and250R⊙(Weigeltetal.;Yudin2002,privatecommunication)in2001,respectively.Thisisafactorof2morethananaverageradiusforanM6/M7IIIstar.Onemustnote,however,thatthemeasuredradiiforcoolgiantsshowlargescatter,andthemeanstandarddeviationsfortheresultingaverageradiiare50%(vanBelleetal.1999).

Recently,Zamanovetal.(2007)havemeasuredtheprojectedrotationalve-locities,vrotsini,forthegiantsin29S-typesymbioticsanddemonstratedthatthesegiantsaresynchronisedwiththeorbitalmotion.Theirresultisconsistentwiththeoreticalstudiesoftidalsynchronizationwhichpredictthesynchroniza-tiontimescalesof∼103–104yrfortypicalparametersofS-typesymbiotics(e.g.Zamanovetal.2007,andreferencestherein).Atpresent,thereare48symbi-oticgiantswithpublishedprojectedrotationalvelocities(Kenyonetal.1991;Miko󰁗lajewska&Kenyon1992;Fekeletal.2003;Hinkleetal.2006;Zamanovetal.2007),and31ofthemalsohaveknownorbitalperiods(Miko󰁗lajewska2003;Gromadzkietal.2007,inpreparation).So,assumingco-rotationwiththeorbitalperiodforallgiantsinS-typesymbiotics,theirradiicanbeinprin-cipleestimatedfromvrotsini.Fig.2acomparestheseradiiwiththeaverageradiicorrespondingtothegiantspectraltypes.Mostofsystemswithknownorbitalperiodsareeclipsingbinaries,andsohavesini≥0.94,andinanycaseareunlikelytohavesini≤0.75(seediscussioninMiko󰁗lajewska2007).Thuswehaveassumedsini=1.InthecaseofBXMonandCD-43◦14304–systemswithsignificantlyeccentricorbits,wehaveadoptedthepseudo-synchronizationperiods(Hut1981)insteadoftheorbitalones.

Generallytheradiiderivedfromvrotsiniagreewiththosepredictedbyspec-traltype.However,insomesystemstheyaresignificantlylarger,althoughcon-sistentwiththetidal(Roche-lobe)radii.Theopticalmeasurementsofvrotsinigivesystematicallylargervaluesthanthenear-IR,andtherearelargedifferencesinsomesystemswithmorethanonemeasurement.

The’rotational’radiiofthecoolcomponentsofbothSyRNedeviatesignif-icantlyfromthevaluespredictedbytheirspectraltypes.InthecaseofRSOph,eitherthegiantrotatesfasterthantheorbitalperiod(Zamanovetal.2007)oritisfillingitstidallobe.InTCrB,thegiantindeedfillsitstidallobe,anditsradiusresultingfromthelightcurvesynthesisisafactorof∼2largerthanthe

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Figure2.(a)Comparisonoftheredgiantradiiderivedfromvrotsini(as-sumingsini=1)withtheaverageradiipredictedfromtheirspectraltypes.Openandfilledcirclesrepresentthevaluesderivedfromtheopticalandnear-IRdata,respectively.Thesolid,dashedanddottedlinescorrespondtoR(vrotsini)=R(Sp)fori=90,60and45◦,respectively.InthecaseofTCrB,wehaveadoptedtheradiusderivedfromlightcurveanalysis(Bel-czy´nski&Miko󰁗lajewska1998)insteadofthatpredictedbythespectraltype.(b)SymbioticgiantsintheHRdiagram.OpenandfilledcirclescorrespondtotheGalacticyellowandremaingsystems,respectively.TheMCsystemsareplottedasopen(SMC)andfilled(LMC)stars.TheSyRNeareplottedasopendiamonds.TheupperandlowerpositionofRSOphcorrespondstotheRoche-lobefillinggiants,andtotheredgiantatd=1.4kpc,respectively(seetext).Evolutionarytracksforlow-massAGBstars(dashedanddottedlinesforZ=0.002andZ=0.02,respectively)andRGBstars(solidlines)arealsoplotted.

’rotational’radius.Asimilareffectisobservedinafewothersymbioticswithtidallydistortedgiants–seebelow(Sec.5).

Theredgiantradiicombinedwiththeeffectivetemperaturesfromthespec-traltypehavebeenusedtocalculatetheluminosity,andtoplotthesymbi-oticgiantsinanHRdiagram(Fig.2b).InthecaseoftheMC’ssymbioticsthedistancesareknown,andtheirluminositieshavebeenestimatedfromKmagnitudes.BoththegalacticandtheMCsymbioticgiantsarelow-mass,Mg<∼2−3M⊙,objects,eitherontheAGB–intheMCsystemsandyellowGalacticsystemswithPorb>∼900d,oratthetopofRGBorthebottomoftheAGBintheremainingsystems(seealsodiscussioninMiko󰁗lajewska2007).ThepositionofthecoolcomponentTCrBintheHRdiagramhasbeenestimatedassumingaRochelobefillinggiant,anditisconsistentofalowmass,<∼1M⊙,solarmetallicitygiant.InthecaseofRSOph,theupperpositioncorrespondstotheRochelobefillinggiant(whichisconsistentwithitsrotationalveloc-ity)whereastheloweroneisconsistentwiththecommonlyaccepteddistance,d∼1.4kpc(Richetal.2008).Inbothcases,thelocationofRSOphintheHRdiagramisconsistentwithalowmass,<∼1M⊙,giantonlyifitsmetallicityissignificantlysubsolar.

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Metalpoorgiantswith[Fe/H]<∼−1arepresentintheyellowsymbioticsystems(e.g.Miko󰁗lajewska2007,andreferencestherein),andtheyallbelongtotheGalactichalopopulation.So,isRSOphanotheryellowsymbioticstar?Scottetal.(1994)foundthatnear-IRspectraofRSOphindicateverylow[C/H]

1213<∼−3andC/C=10,andthesevaluesareverysimilartothoseestimatedfor

metal-poor([Fe/H]∼−2)fieldhalogiants(e.g.Keller,Pilachowski&Sneden2001).However,recentstudiesoftheopticalspectraofTCrBandRSOphhavegivenanearlysolarmetallicity(Wallerstein,Harrison&Munari2006).TheonlychemicalanomalydetectedinthesetwoSyRNeisthelithiumoverabundance(Shabhazetal.1999;Wallersteinetal.2006).SuchhighLiabundancesarecommoninlate-typesecondariesinneutronstarandbackholebinaries(e.g.Martinetal.1994)butextremelyrareinthesymbioticgiants.Infact,theLienhancementisdetectedonlyinthesymbioticMiraV407Cyg,whereitcanprobablybeexplainedasaconsequenceofhotbottomburning,whichoccursinstarswithinitialmassesintherange4–6M⊙(Tatarnikovaetal.2003).Theverylongpulsationperiod,P=745days,supportsthisinterpretationinthecaseofV407CygassimilarLienhancementshavebeenfoundinLMCmiraswithverylongperiods(>∼400days),andinonegalacticmirawithcomparableperiod.Asimilarexplanationis,however,unlikelyinthecaseofthelowmass,<∼1M⊙,nonpulsatinggiantsintheSyRNe.ThefactthattheLienhancementisacommonfeatureofthesecondariesinbothlow-massX-raybinaries(LMXRB),andtheSyRNe,indicatesthattheremustbeaprocessofLiproductionoperatinginsuchbinarysystems,andindependentlyoftheverydifferentnatureoftheircompactcompanions(ablackholeofaneutronstarinLMXRB,andamassivewhitedwarfintheSyRNe).5.

Masstransfermodeandaccretion

Amongthemostfundamentalquestionsposedbythesymbioticbinariesisthemodeofmasstransfer–Roche-lobeoverfloworstellarwind,andthepossibilityofaccretiondiscformation.

3-Dhydrodynamicalmodelshaveshownthatintypicalsymbioticbinariesthecoolgiantwindissignificantlydeflectedtowardstheorbitalplanebythegravitationalpullofitscompanion(e.g.Mastrodemos&Morris1998,1999;Gawryszczak,Miko󰁗lajewska&R´oz˙yczka2002,2003)whichwouldalsofacilitatetheformationofanaccretiondiscaroundthecompanion.Suchapictureissupportedbyobservations.Inparticular,resolvednebulaeinD-typesymbioticsshowabipolargeometrywhichisbestaccountedbyintrinsicasphericityofthewindand/oreffectsassociatedwiththepresenceofanaccretiondiscaroundthehotcomponent.Thereisalsostrongspectroscopicevidenceforfast,>∼

−11000kms,jetsandbipolaroutflowsinactiveS-typesymbiotics(e.g.Tomov

2003)includingZAnd(Burmeister2008)mostlikelyproducedinanaccretiondiscenvironment.So,atleasttransientaccretiondiscsseemtobecommonlyformedinthesebinaries.

However,tillrecently,mostresearchershavefavoredwindaccretionovertheRoche-lobeoverlow.WhereasthisstandpointisoutofquestioninthecaseofD-typesystems,thesituationinS-typesystems,especiallythosewithshorter,

d<∼1000,orbitalperiods,islessobvious.Infact,themainargumentsagainstthe

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Roche-lobeoverflowhavebeentheredgiantradiiderivedfromtheirprojectedrotationalvelocities,andthelackofevidenceforellipsoidalvariabilityintheirlightcurves.ThelatterargumentisnottrueanymoreasthereisclearevidenceforellipsoidallightcurvevariabilityinmanyS-typesystemswhichwouldsuggestthatthemass-losinggiantisfillingitsRochelobeorisatleastveryclosetofillingitsRochelobe.Inparticular,tidallydistortedgiantdonorshavebeendetectedinallsymbioticswithmultipleoutburstZAnd-typeactivitywheneverrelevantredornear-IRlightcurveshavebeenavailable(Miko󰁗lajewska2007).Moreover,inmostofthesymbioticswithellipsoidalvariabilitythereissignificantdiscrepancybetweentheredgiantradiiderivedfromvrotsiniandthoseindicatedbythelightcurveanalysis(seeFig.4ofMiko󰁗lajewska2007,anddiscussiontherein).Althoughthereisnotyetagoodsolutiontothisproblem,oneshouldrememberthatthevrotsiniestimatesforthesetidallydistortedsymbioticgiantsarebiasedbecausetheusuallyadoptedmodelassumesashericalshapeofthegiantandasimplelimb-darkeninglaw(Orosz&Hauschildt2000).

Theellipsoidalvariationsaredefinitelypresentinthequiescentopticalandnear-IRlightcuvesofTCrB,andthelightcurveanalysishaveconfirmedthatthecoolcomponentfillsitstidallobe(Belczynski&Miko󰁗lajewska1998).InthecaseofRSOph,thesituationismorecomplicated.Whilethecoolgiantra-diusinferredfromthevrotsinimeasurementindicatesthatitmayfillitsRochelobe,thequiescentvisuallightcuvesshowverycomplexorbitalbehaviour(Gro-madzki,Miko󰁗lajewska&Lachowicz,2008).Inparticular,thereisalwayspresentaminimumatthetimeofthespectroscopicconjunctionwiththeredgiantinfront,andthedepthcorrelatedwiththesystemaveragebrightness.ThereisalsoamovingbumpsimilartothosefoundinotheractivesymbioticsystemsbutnoevidenceforasecondaryminimumexpectedinthecaseoftheRoche-lobefillinggiant.Oneshouldnote,however,thatthemovingbumpcanveryeasilyveilthesecondaryminimumasitdoesforinstanceinARPavinwhichtheellipsoidalchangesareveryevidentinthenear-IRbands,whereasthevisuallightcurveshowsonlyverydeepprimaryminimumandthebump(Rutkowski,Miko󰁗lajewska&Whitelock2007).Althoughitispossiblethatthecoolcompo-nentofRSOphdoesnotfillitRochelobe,itishardtounderstandwhytheevolutionarystatusandthemasstransfermodeofRSOphdiffersomuchfromthoseinTCrB,especiallygiventhatRSOphismoreactiveanditshotcom-ponentbrigher,andithasamuchshorterrecurrencetimebetweentheTNReruptionsthanTCrB.Thisrequiresahighermasstransfer/accretionrateinRSOphthanthatinTCrB,whichiseasiertoensurebytheRoche-lobeoverflow.6.

Concludingremarks

Themainpointsofthispaperaresummarizedbelow.

•Therecurrentnovaewithredgiantsecondariesaredefinitelysymbioticstarsastheysharemanyphysicalcharacteristicswiththesesystems.•TheorbitalperiodsoftheSyRNe,RSOphandTCrBfallintotheshorterperiodtailofgalacticS-typesystems.ThesetwoSyRNe,andV2116Oph/GX1+4aretheonlysymbioticswiththecoolgiantbeingthelessmassivecomponent,withmass<∼1M⊙inallcases.Thewhitedwarfsin

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theSyRNearethemostmassive,withM∼1.1−1.4M⊙,sufficientforthemtobecomesupernovaeIa.

•Roche-lobeoverflowseemstobequitecommoninS-typesymbioticstars,especiallyinthosewithmultipleoutburstZAnd-typeactivity,andthetidallydistortedredgiantandRoche-lobeoverflowisalsopresentinTCrBandpossiblyinRSOph.•BoththeactivityofZAnd-typesymbioticsaswellasthehighandlowstatesobservedintheSyRNebetweentheirTNRnovaeruptionsareduetounstabledisc-accretionontowhitedwarf.HoweverthewhitedwarfsinZAnd-typesystemsburntheaccretedhydrogenmoreorlessstablywhereasintheSyRNetheydonot.Acknowledgments.01727.

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