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Paper text:
m15 is a massive core - collapsed cluster , with an enormous stellar density in the central @xmath01@xmath1@xmath21@xmath1 , reflected in the scarcity of photometric variable star detections in this field .
however , ferraro & paresce ( 1993 ) used the hst / foc to identify 19 variable candidates . our earlier work ( butler et al . 1998 ) , using a triffid / mama camera similar to that described here , resulted in light curves and periods for all but four of these , with a further 16 suspected new variables .
we made new observations in _ b _ and _ v _ simultaneously with the triffid-2/mama camera on the 1 m jkt ( la palma ) , over 12 nights in july 1997 , with seeing of 0@xmath37 - 0@xmath38 .
the 2-d photon - counted data were first sharpened post - exposure by tip / tilt correction at 1ms time resolution . the isis ( alard & lupton 1998 ) image matching and difference imaging package was used to search for frame - to - frame variability .
all star - like objects 6-sigma above the local background of the the _ median _ of the set of difference images were selected as candidate variables ( see figure 1 ) .
psf - fitting photometry was performed at accurate positions for the candidate variables in the difference images .
the resulting relative fluxes were converted to absolute fluxes by performing profile - fitting photometery on the reference image .
the instrumental magnitudes were zeropointed against photometry of hst / wfpc2 archival images in the f555w and f439w filter system .
accurate periods ( and hence lightcurves ) were obtained for the set of magnitude measurements for all candidate variables using the phase dispersions minimisation technique ( pdm , stellingwerf 1978 ) .
simulations performed using an artificial image - set modeled on our m15 observations indicate that we are sensitive to rr lyrae variables with an amplitude swing of 0.15 mag ( at 80% detection probability ) .
we also increased our sensitivity to dwarf novae ( dne ) by summing all images per night prior to isis analysis .
we confirmed all known variables and detected 22 new ones . the lightcurve shape , amplitude , scatter and most significant period distinguished 12 rrab , 19 rrc and 2 ( probable ) rrd variables .
the nature of 11 variables remains uncertain .
we found that the core rr lyrae population has similar periods and amplitudes to the outer population .
however , in the core we find a marked increase in the ratio of rrc / rrab stars and a dearth of rrd stars , with respect to the outer reaches .
the latter can be reconciled on observational grounds but the former can not .
also detected were the lmxb counterpart ac211 , a w vir type cepheid , and a bl her type cepheid - objects 20 , 33 and 18 in figure 1 .
our search yielded no candidate dne . from simulations , we estimate fewer than 10 ( with 92% probability )
dne of absolute mag 4.3 ( f555w/@xmath4 ) to exist in the studied region .
this is the first such quantitative result for m15 .
a two - week campaign of high - resolution imaging of the centre of m15 using the tip / tilt correcting triffid-2 camera , followed by analysis with isis ( image matching & subtraction software ) , has produced the most sensitive survey to date of variable stars in the central @xmath01@xmath1@xmath21@xmath1 - a total of 48 were detected - , and constrains the size of the dwarf nova population .    
 # 1_#1 _ # 1_#1 _ =    # 1 1.25 in .125 in .25 in