THE RR LYRAE VARIABLES AND HORIZONTAL BRANCH OF NGC 6656 (M22)

The first calibrated broadband UBVI time-series photometry is presented for the RR Lyrae variable stars in NGC 6656 (M22), with observations spanning a range of 22 years. We have also redetermined the variability types and periods for the RR Lyrae stars identified previously by photographic observations, revising the number of fundamental-mode RR Lyrae variables (RR0) to 10 and the number of first-overtone variables (RR1) to 16. The mean periods of the RR0 and RR1 variables are [left angle bracket]P[right angle bracket] sub(RRO) = 0.66 + or - 0.02 days and [left angle bracket]P[right angle bracket] sub(RR1) = 0.33 + or - 0.01 days, respectively, supporting an Oosterhoff II classification for the cluster. The number ratio of RR1-type to all RR-type variables is N sub(1)/N sub(RR) = 0.61, also consistent with an Oosterhoff II designation. Both the RR Lyrae stars' minimum light colors and the blue edge of the RR Lyrae instability strip suggest E(B - V) = 0.36 + or - 0.02 mag toward M22. Regarding the HB morphology of M22, we find (B-R)/(B+V+R) = +0.97 + or - 0.1 and at least one "gap" located in an unusual part of the blue HB, in the middle of the so-called hot HB stars.