High-speed colourimetry of the subdwarf B star SDSS J171722.08+58055.8 with ULTRACAM

We present high-speed multicolour photometry of the faint V 361 Hya star SDSS J171722.08+58055.8 (B ≃ 16.7), which was recently discovered to be a pulsating subdwarf B star. The data were obtained during two consecutive nights in 2004 August using the three-channel photometer ULTRACAM attached to the 4.2-m William Herschel Telescope. The data have a total time-span of 1.11 d and consist of two groups spanning roughly 2.5 h each. The adopted integration time was 10 s. We confirm the star to be oscillating and we refine the dominant frequency to 6.960 ± 0.022 mHz. A second new oscillation frequency of 7.267 ± 0.025 mHz is discovered, having a well-covered beat period of 0.9 h with the dominant one. Both modes have amplitudes that are significant in all three colours at a level >5σ and show, within the measurement accuracy, the same phase in all three colours. We attempted mode identification for the two modes from their amplitude ratios but did not obtain conclusive results due to the too-large uncertainties in the observed ratios.