Discovery and analysis of p-mode and g-mode oscillations in the A-type primary of the eccentric binary HD 209295*

We have discovered both intermediate-order gravity mode and low-order pressure mode pulsation in the same star, HD 209295. It is therefore both a γ Doradus and a δ Scuti star, which makes it the first pulsating star to be a member of two classes. The analysis of our 128h of multisite spectroscopic observations carried out over two seasons reveals that the star is a single-lined spectroscopic binary with an orbital period of 3.10575+/-0.00010d and an eccentricity of 0.352+/-0.011. Only weak pulsational signals are found in both the radial velocity and line-profile variations, but we have succeeded in showing that the two highest-amplitude γ Doradus pulsation modes are consistent with l=1 and |m|=1. These two modes dominated our 280h of BVI_C multisite photometry, also obtained over two seasons. We detected altogether ten frequencies in the light variations, one in the δ Scuti regime and nine in the γ Doradus domain. Five of the γ Doradus frequencies are exact integer multiples of the orbital frequency. This observation leads us to suspect they are tidally excited. Attempts to identify modes from the multicolour photometry failed. We performed model calculations and a stability analysis of the pulsations. The frequency range in which δ Scuti modes are excited agrees well with observations. However, our models do not show excitation of γ Doradus pulsations, although the damping is smaller in the observed range. We also investigated tidal excitation of γ Doradus modes. Some of the observed harmonics of the orbital period were found to be unstable. The observed orbital harmonics which are stable in the models can be understood as linear combinations of the unstable modes. We could not detect the secondary component of the system in infrared photometry, suggesting that it may not be a main-sequence star. Archival data of this star show that it has a strong ultraviolet (UV) excess, the origin of which is not known. The orbit of the primary is consistent with a secondary mass of M>1.04M_solar, which is indicative of a neutron star, although a white dwarf companion is not ruled out.