HD 51844: An Am δ Scuti in a binary showing periastron brightening

Context. Pulsating stars in binary systems are ideal laboratories to test stellar evolution and pulsation theory, since a direct, model-independent determination of component masses is possible. The high precision CoRoT photometry allows a detailed view of the frequency content of pulsating stars, enabling detection of patterns in their distribution. The object HD 51844 is such a case showing periastron brightening instead of eclipses. Aims. We present a comprehensive study of the HD 51844 system, where we derive physical parameters of both components, the pulsation content and frequency patterns, Additionally, we obtain the orbital elements, including masses, and the chemical composition of the stars, Methods, Time series analysis using standard tools was employed to extract the pulsation frequencies. Photospheric abundances of 21 chemical elements were derived by means of spectrum synthesis. We derived orbital elements both by fitting the observed radial velocities and the light curves, and we did asteroseismic modelling as well. Results. We found that HD 51844 is a double lined spectroscopic binary. The determined abundances are consistent with delta Delphini classification. We determined the orbital period (33498 +/- 0.002 d). the eccentricity (0.484 +/- 0.020), the mass ratio (0.988 +/- 0.02). and the masses to 2.0 +/- 0.2 M-circle dot for both components. Only one component showed pulsation. Two p modes {f(22). and f(36)) and one g mode (f(orb)) may be tidally excited. Among the 115 frequencies, we detected triplets due to the frequency modulation, frequency differences connected to the orbital period, and unexpected resonances (3:2, 3:5, and 3:4), which is a new discovery for a delta Set star. The observed frequency differences among the dominant modes suggest a large separation of 2.0-2.2 d(-1), which are consistent with models of mean density of 0.063 g cm(-5), and with the binary solution and TAMS evolutionary phase for the pulsating component. The binary evolution is in an intermediate evolutionary phase; the stellar rotation is super-synchronised, but circularisation of the orbit is not reached.