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the cosmic lens all - sky survey ( class ; @xcite ) is the largest statistically complete gravitational lens survey ever undertaken .
a complete sample of 11  685 flat - spectrum radio sources ( @xmath0 where @xmath1 between 1.4 and 4.85  ghz ) was observed with the very large array ( vla ) in a configuration at 8.46  ghz between 1994 and 1999 .
the 0.2-arcsec angular resolution provided by the vla allowed the straight forward identification of galaxy - scale gravitational lenses , which typically have image separations of @xmath2  arcsec .
class sources were identified as promising gravitational lens candidates if the vla 8.46  ghz image contained multiple compact components with gaussian fwhm @xmath3  mas , the image separations were between @xmath4  arcsec , the total integrated flux - density was @xmath5  mjy and the component flux - density ratio was @xmath6 .
the detection of compact structure and extended emission in the multiple components at higher resolutions with merlin ( multi - element radio linked interferometer network ) and the vlba ( very long baseline array ) confirmed the lensing hypothesis for 21 gravitational lenses during the first three phases of class ( see @xcite and references therein for a discussion of all the gravitational lenses discovered by class ) .    in this paper , we present observations of the gravitational lens candidate class b2108 + 213 , which was discovered during the fourth and final phase of class . in section [ radio ]
we describe the vla , merlin and vlba radio observations which led to the discovery of the gravitational lens candidate class b2108 + 213 .
follow - up optical and infrared imaging of the system and the surrounding field , taken with the _ hubble space telescope _ ,
is presented in section [ photometry ] . using the available observational data , the lensing hypothesis , along with an analysis of the lensing potential , is discussed in section [ disc ] .
finally , in section [ conc ] we summarise our findings and outline the future work we intend to carry out on this intriguing new gravitational lens from class . for all calculations we adopt an @xmath7 , @xmath8 flat - universe , with a hubble parameter of @xmath9  km  s@xmath10 mpc@xmath10 .
class b2108 + 213 was observed with the vla in a configuration at 8.46  ghz as part of the final phase of class on 1999 august 17 .
the total on - source integration time was @xmath11 seconds .
a 3.3 second correlator integration time with two 50  mhz ifs were used .
3c286 was chosen as the primary flux - density calibrator and phase - referencing was carried out with jvas b2059 + 034 ( jodrell bank vla astrometric survey ; @xcite ) .
the data were edited and calibrated in the standard way using aips ( astronomical image processing software ) and imaged within the caltech vlbi difference mapping package ( difmap ; @xcite ) using the class mapping script @xcite .
the @xmath12-data were naturally weighted and elliptical gaussian model components were fitted to each image .
the rms noise level was 180  @xmath13jy  beam@xmath10 and the beam size was @xmath14  arcsec@xmath15 .
the vla detected three compact components , shown in fig .
[ vla - map ] , whose flux - densities and relative positions are given in table [ data - tab ] and table [ 2108-relative ] , respectively . with an image separation between components
a and b of 4.56  arcsec , class b2108 + 213 has a wider image separation than any gravitational lens system thus far discovered by class ( see fig .
[ image - sep ] for the class gravitational lens image separation distribution ) .
the total integrated flux - density of the system at 8.46  ghz is 18.5  mjy and the flux - density ratio between the brightest and faintest components is @xmath16 .
therefore , class b2108 + 213 failed to meet the strict lens candidate selection criteria due to its total integrated 8.46  ghz flux - density being less than 20  mjy .
however , class b2108 + 213 was followed up as a promising lens candidate even though it was no longer part of the statistically well defined class sample .
( 6,7.6 ) ( -0.9,-0.0 )    [ cols="<,<,>,>,>,>,>,>,^ " , ]
we have presented radio and optical observations of the new gravitational lens class b2108 + 213 .
this system comprises two lensed images , which are separated by 4.56  arcsec , thus making class b2108 + 213 the widest separation gravitational lens discovered by class .
optical imaging found two lensing galaxies within the einstein radius of the system .
intriguingly , we also detected radio emission from a third component which was coincident with the optical emission from the primary lensing galaxy .
the extension of the third radio component and its different radio spectrum from the two lensed images suggests that it is unlikely to be a third lensed image .
however , it is too early to completely discount the possibility that scatter broadening effects might have been responsible for modifying the properties of a third image .
the positions and flux - densities of the lensed images were successfully reproduced by a model containing two singular isothermal spheres and an external shear . however , further observational constraints will need to be obtained for a detailed analysis of the class b2108 + 213 lensing potential to be carried out .
the wide image separation of class b2108 + 213 implies a group / cluster assisted lensing potential .
therefore , class b2108 + 213 provides an excellent opportunity to study the structure and mass distribution of galaxy groups beyond the local universe ( @xmath17 ) .
we have begun a spectroscopic programme to determine the redshifts of the lensing galaxies and lensed source in class b2108 + 213 , and to identify which nearby galaxies are part of the lensing group .
thus far we have found four nearby galaxies at similar redshifts to g1 ( @xmath18 ; mckean et al .
, in preparation ) . furthermore , our spectrum of g1 has sufficient signal - to - noise for the stellar velocity dispersion to be measured .
these observations will provide additional constraints for a future mass model .
the other interesting feature of class b2108 + 213 is the possible detection of a third lensed image .
although the observations presented here show that this is almost certainly not the case , this conclusion could be confirmed by measuring the flux - density , polarization and size of the radio emission from each component as a function of frequency .
if scattering of a third lensed image is occurring then we would expect the high frequency spectrum of each radio component to be identical .
the vla and vlba are operated by the national radio astronomy observatory which is a facility of the national science foundation operated under cooperative agreement by associated universities , inc .
merlin is a national facility operated by the university of manchester on behalf of pparc .
the results present herein were based on observations collected with the nasa / esa _ hst _ , obtained at stsci , which is operated by aura , under nasa contract nas5 - 26555 .
jpm , man , pmp and tdy acknowledge the receipt of pparc studentships .
rdb is supported by nsf grant ast-9900866 .
tt acknowledges support from nasa through hubble fellowship grant hst - hf-01167.01-a .
this work was supported by the european community s sixth framework marie curie research training network programme , contract no .
mrtn - ct-2004 - 505183 `` angles '' .
we present observations of class b2108 + 213 , the widest separation gravitational lens system discovered by the cosmic lens all - sky survey . 
 radio imaging using the vla at 8.46  ghz and merlin at 5  ghz shows two compact components separated by 4.56  arcsec with a faint third component in between which we believe is emission from a lensing galaxy . 
 5-ghz vlba observations reveal milliarcsecond - scale structure in the two lensed images that is consistent with gravitational lensing . 
 optical emission from the two lensed images and two lensing galaxies within the einstein radius is detected in _ 
 hubble space telescope _ imaging . 
 furthermore , an optical gravitational arc , associated with the strongest lensed component , has been detected . 
 surrounding the system are a number of faint galaxies which may help explain the wide image separation . a plausible mass distribution model for class b2108 + 213 is also presented .    [ firstpage ]    
 quasars : individual : class b2108 + 213 , cosmology : observations , gravitational lensing .