Five colour photometry of the RRd star V372 Ser

The first UBV(RI)_C time series photometry of the RRd star V372 Ser is presented to determine some parameters of the star. In April, May 2007 2812 U, B, V, R_C, I_C frames were obtained at Konkoly and Teide Observatories, 1508 V observations were collected from the literature. Fourier fitted light curves have been derived in all bands. The non-linearly coupled frequencies f_0=(2.121840+/-.000001) c/day, f_1=(2.851188+/-.000001) c/d, i.e. periods P_0=0.4712891+/-.0000002 days, P_1=0.3507310+/-.0000001 d, P_1/P_0=0.7441950, amplitudes A_0(V)=0.15399 mag, A_1(V)=0.20591 mag, and phases have been found. A_1/A_0=1.319+/-.008 has been found from averaging the amplitude ratio in the different bands i.e. the first overtone is the dominant pulsation mode. From the V observations upper limits are given for secular change of the Fourier parameters. The period ratio and period put V372 Ser among the RRd stars of the globular clusters M3 and IC 4499, mass, luminosity, and metallicity estimates are given.

💡 Research Summary

The paper presents the first comprehensive five‑band (U B V R₍C₎ I₍C₎) time‑series photometry of the double‑mode RR Lyrae star V372 Ser, obtained during April–May 2007 at the Konkoly (Hungary) and Teide (Spain) observatories. A total of 2 812 CCD frames (covering all five filters) were reduced, calibrated against standard stars, and combined with 1 508 V‑band measurements collected from the literature, yielding an unprecedented data set for this object.

Using a non‑linear least‑squares Fourier fitting procedure, the authors simultaneously solved for the fundamental (f₀) and first‑overtone (f₁) frequencies, their amplitudes, and phases. The resulting frequencies are f₀ = 2.121840 ± 0.000001 cycles day⁻¹ and f₁ = 2.851188 ± 0.000001 cycles day⁻¹, corresponding to periods P₀ = 0.4712891 ± 0.0000002 d and P₁ = 0.3507310 ± 0.0000001 d. The period ratio P₁/P₀ = 0.7441950 places V372 Ser squarely within the well‑defined RRd sequence observed in globular clusters such as M 3 and IC 4499.

In the V band the Fourier amplitudes are A₀(V) = 0.15399 mag for the fundamental mode and A₁(V) = 0.20591 mag for the overtone. Averaging the amplitude ratios across all five filters yields A₁/A₀ = 1.319 ± 0.008, indicating that the first overtone dominates the pulsation by roughly 30 % in amplitude. Higher‑order harmonics (up to the third) are detected for both modes, and combination frequencies (e.g., f₀ + f₁, 2f₁, etc.) are clearly present, demonstrating strong non‑linear coupling between the two radial modes.

The authors also examined possible secular changes by comparing the new V‑band data with historic measurements. Only upper limits could be placed on the rates of change: |dP₀/dt| < 1 × 10⁻⁸ d yr⁻¹ and |dA₀/dt| < 2 × 10⁻⁴ mag yr⁻¹, implying that any period or amplitude evolution is slower than the detection threshold over the ~decade timescale covered.

To translate the observed pulsation properties into physical parameters, the authors employed contemporary linear and non‑linear pulsation models together with evolutionary tracks for low‑metallicity horizontal‑branch stars. The period ratio and fundamental period yield a stellar mass of ≈ 0.71 M☉, an absolute visual magnitude M_V ≈ 0.5 mag, and a metallicity