Physical properties of IP Pegasi: an eclipsing dwarf nova with an unusually cool white dwarf

We present high-speed photometric observations of the eclipsing dwarf nova IP Pegasi (IP Peg) taken with the triple-beam camera ULTRACAM mounted on the William Herschel Telescope. The primary eclipse in this system was observed twice in 2004, and then a further 16 times over a 3 week period in 2005. Our observations were simultaneous in the Sloan u', g' and r' bands. By phase-folding and averaging our data, we make the first significant detection of the white dwarf ingress in this system and find the phase width phi of the white dwarf eclipse to be 0.0935 +/- 0.0003, significantly higher than the previous best value of 0.0863 < phi < 0.0918. The mass ratio is found to be q = M-2/M-1 = 0.48 +/- 0.01, consistent with previous measurements, but we find the inclination to be 83 degrees.8 +/- 0 degrees.5, significantly higher than previously reported. We find the radius of the white dwarf to be 0.0063 +/- 0.0003 R-circle dot, implying a white dwarf mass of 1.16 +/- 0.02 M-circle dot. The donor mass is 0.55 +/- 0.02 M-circle dot. The white dwarf temperature is more difficult to determine, since the white dwarf is seen to vary significantly in flux, even between consecutive eclipses. This is seen particularly in the u' band, and is probably the result of absorption by disc material. Our best estimate of the temperature is 10 000-15 000 K, which is much lower than would be expected for a cataclysmic variable star with this period, and implies a mean accretion rate of < 5 x 10(-11) M(circle dot)yr(-1), more than 40 times lower than the expected rate.