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Paper text:
there is no general concensus yet about what shapes planetary nebulae ( pne ) .
the idea that the presence of a stellar or substellar companion is needed to break the spherical symmetry and account for the observed geometries , called the binary hypothesis ( see @xcite ) , is still being tested .
the fact that 80% of pne are non - spherical ( @xcite ) potentially implies that the proportion of binary central stars of pne ( cspn ) is much larger than current estimates , therefore indicating that pne could be preferentially created via a binary channel . if the observed fraction of binary cspn were higher than expected in the current paradigm ( 35% for binaries with separations
@xmath1 au ) , this would support the idea that pne are preferentially a binary phenomenon . to estimate the binary fraction ,
we need an unbiased sample of pne . the determination of whether a cspn is binary is not trivial and requires a suite of methods , which we describe below
. we also present new obervational results and how this constrains the binary fraction .
we conclude with some thoughts to refine this value .
we here present the methods used to investigate binarity in our pn sample : flux and spectral variability and infrared ( ir ) excess , as well as recent results obtained with them .
the binary - induced flux variability method is based on the variation of brightness of the binary system .
the three main causes of this flux variability are eclipses , tidal deformations , but mainly irradiation effects from the hot companion onto the cold one create flux changes .
this method gives a close binary fraction of about 15 - 20% ( see @xcite , @xcite ) .
it is biased to short periods as the three causes of binary induced flux variation all diminish with increasing separation .
it is also biased to intrinsically faint stars to avoid periodic variability caused by stellar winds .
pulsations can also be accountable for flux variation .
new discoveries will be possible thanks to kepler , whose field includes 6 pne , including kronberger 61 , newly imaged by us in march 2011 ( see http://www.gemini.edu/node/11656 ) .
five of the six objects show flux variability ( see @xcite . )    spectroscopic variability is used to detect the motion of binary systems around their barycenter .
binaries with periods up to a month could potentially be detected with this method .
intrinsically faint stars , with weak or no wind , are targets of choice .
this introduces a bias and imposes serious technical restrictions to obtain a sufficient signal - to - noise ratio , ranking this method down for statistical purposes required in our case . out of 7 central stars targeted by the vlt / uves
, we found that one , a 14 , has a radial velocity ( rv ) variability confirmed at a 3@xmath2 level .
another 3 central stars are likely to be binaries ( douchin et al .
, in preparation ) .
the ir excess technique detects the ir emission from a cool companion using high precision optical and nir photometry .
so far , the detection limit is an m8-type star for intrinsically faint central stars ( mv = 6 - 8 ) .
this technique has the advantage that it is less biased by binary separation .
we aim to perform high precision photometry for the entire 2 kpc volume - limited sample of frew ( @xcite ; @xcite ) .
so far , out of 28 central stars of pne that were observed in the i - band , we have solidly detected an i - band excess in 3 , with another possible 5 detections .
the spectral types for the 3 detected companions are likely in the early m regime .
we have not yet quantified our survey s bias against fainter companions ( passay et al .
, in preparation ) .
we are currently analysing additional i and j - band photometry datasets which will provide us a preliminary estimate of the binary fraction ( separations less than 500 au ) .
in addition , space - based observations such as gaia have a potential capability to detect the reflex orbital motion of central stars of pne for binaries with intermediate separations , and the extreme photometric precision of kepler can be used to gauge how many close binaries have avoided detection in previous ground based surveys ( see @xcite ) . also , future elts will have the resolving power and sensitivity to detect companions via ao - enhanced direct imaging at separations of a few au and larger out to 2 kpc , to provide good complementarity to current methods .
during the past 20 years , the idea that non - spherical planetary nebulae ( pn ) may need a binary or planetary interaction to be shaped was discussed by various authors . 
 it is now generally agreed that the varied morphologies of pn can not be fully explained solely by single star evolution . 
 observationally , more binary central stars of planetary nebulae ( cspn ) have been discovered , opening new possibilities to understand the connections between binarity and morphology . 
 so far , @xmath045 binary cspn have been detected , most being close systems detected via flux variability . to determine the pn binary fraction 
 , one needs a method to detect wider binaries . 
 we present here recent results obtained with the various techniques described , concentrating on binary infrared excess observations aimed at detecting binaries of any separation .