  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. 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.
Time series analysis using standard tools was mployed 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. 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 (33.498 +- 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_sun 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
Scuti star.
