HD 143418: an unusual light variable and a double-lined spectroscopic binary with a CP primary

Aims. After a serendipitious discovery that HD 143418 is a light variable, we analyzed numerous UBV observations and Hipparcos H-p magnitudes transformed to Johnson V to find out whether the object is a very unusual spectroscopic binary. Methods. Initial reductions of new photometry were carried out with the HEC22 program, while the new spectra were reduced in the IRAF and SPEFO programs. Orbital elements were derived with the FOTEL program, the KOREL disentangling was applied, and period searches were carried out using phase dispersion minimalization technique and the program PERIOD04. Final modeling of radial-velocity and light variations was carried out in PHOEBE. Results. The brightness of HD 143418 varies with a period of 2(d)282520 +/- 0(d).000010 and with little color changes. The light curve has a non-sinusoidal shape, which moreover changes from season to season. In each season, it can be described by the basic frequency and its first harmonic. Its long-term variations are characterized by cyclic variations of the amplitudes of the two frequencies and also the phase of the first frequency, the phase of the first harmonic remaining secularly constant. The amplitude of the principal frequency varies over a larger range and in antiphase to that of the first harmonic. Radial-velocity variations with the same period identify the object as a double-lined spectroscopic binary. The secondary spectrum is very faint, however. The rotation of the CP primary is strongly subsynchronous (its probable rotation period is 6.(d)8 +/- 0.(d)2). We show that the observed light variations can be understood as a combination of the ellipsoidal variability in the binary system and either a secularly varying pattern of spots on the secondary or an inhomogeneous corotating cloud ejected from the primary. Conclusions. Obviously, HD 143418 is an astrophysically interesting and dynamically unusual system that deserves future systematic spectral and photometric monitoring and theoretical modeling.