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THE RR LYRAE VARIABLES AND HORIZONTAL BRANCH OF NGC 6656 (M22)<sup>,</sup>
Kunder, Andrea,Stetson, Peter B.,Cassisi, Santi,Layden, Andrew,Bono, Giuseppe,Catelan, Má,rcio,Walker, Alistair R.,Paredes Alvarez, Leonardo,Clem, James L.,Matsunaga, Noriyuki,Salaris, Maurizio American Institute of Physics 2013 The Astronomical journal Vol.146 No.5
<P>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 〈P〉<SUB>RR0</SUB> = 0.66 ± 0.02 days and 〈P〉<SUB>RR1</SUB> = 0.33 ± 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</SUB>/N<SUB>RR</SUB> = 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 ± 0.02 mag toward M22. Regarding the HB morphology of M22, we find (B-R)/(B+V+R) = +0.97 ± 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.</P>