Two double-mode RR Lyrae stars in the field
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Two new double-mode (RRd) field variables are identified in the Lick RR Lyrae survey field (RR VIII) in Draco. Only one field RRd star (AQ Leo) was previously known although some thirty are known in globular clusters. For variable RR VIII-58, P0 = 0d.464718 and P1 = 0d.345389; for variable RR VIII-10, P0 = 0d.511844 and P1 = 0d.381275. The mean B magnitudes of these stars are 14.9 and 16.5, respectively. The amplitudes of the two modes are comparable for RR VIII-58, but the amplitude of the primary (first-overtone) period is significantly greater in RR VIII-10. The mean B - V colors (0.29 and 0.28) are intermediate between those found for RR Lyrae stars of type c and type ab in this field. The theoretical mass calibration of the P1/P0 versus P0 (Petersen) diagram by Cox et al. was used to derive masses of 0.49, 0.58, and 0.66 M for VIII-58, VIII-10 and AQ Leo, respectively. The star AQ Leo lies in an area of the Petersen diagram occupied by the RRd stars in Oosterhoff type II clusters, but the two new field RRd stars lie in areas that are not occupied by cluster RRd stars. [Fe/H] estimates of -1.90, -1.91, and -1.81, respectively were derived by the S method using the calibration of Zinn & West. (The S for AQ Leo was taken from Mendes de Oliveira & Smith). In all cases the abundance data were reduced with the method used for RRc type variables, and the implications of this are discussed. The difference between the H and the mean of the Hy and H equivalent widths is also shown to be proportional to [Fe/H]; this relation is used to derive [Fe/H] values (of lower weight) of -1.4 and -1.7, respectively, for VIII-58 and VIII-10. These results give no evidence for a mass-metallicity relationship among these three RRd field stars, but, in the mean, they agree with the mass-metallicity relation given by the cluster RRd stars.