Double-mode RR Lyrae stars in the globular cluster IC 4499.

Thirteen double-mode RR Lyrae (RRd) stars, with mean magnitudes >B<_c = 18.30±0.10 mag and >V<_c = 17.80±0.15 mag, have been identified in the variable rich Oosterhoff type I globular cluster IC 4499. The stars have surprisingly uniform properties, and are considerably different from RRd stars found in Oosterhoff type II systems. The mean first-overtone period is >P₁< = 0^d.357 with σ = 0^d.005 (cf. >P₁< = 0^d.40 for OoII RRd stars), and the mean ratio of the first-overtone period to the fundamental period is >P₁/P₀< = 0.7444±0.0002. The mean double-mode pulsation mass for the 13 stars is 0.538±0.004 M_sun;. Such an average mass is ≡0.11 M_sun; smaller than that for RRd stars found in OoII systems. The metal abundances for the RRd stars, and for the system of RR Lyrae stars as a whole, are found to be consistent with >[Fe/H]< = -1.38±0.20, determined from ΔS spectroscopy. In the Petersen diagram, all known RRd stars now divide into two groups (split according to Oosterhoff type). With a reddening of E_B-V = 0.27±0.03 mag, the cluster distance modulus is (m-M)₀ = 16.20±0.23 mag.