The radial velocities and physical parameters of HD 553

ER Vul is an eclipsing binary consisting of two solar type stars in a very close orbit with a period of 0: d 7. Accordingly, the two stars rotate very fast, leading to a blending of many spectral features at all phases. Therefore, measuring the radial veloc- ity curve without systematic errors is not trivial. Here, we use a two-dimensional cross-correlation method applied to 137 high quality spectra, collected over 3 years, in order to obtain the radial velocity curve and determine the orbital and some phys- ical parameters of the system from it. Primarily, we improve the binary period to 0: d 69809458 0: 00000014, and find that the two amplitudes are slightly smaller than those measured by others, while the mass ratio is still similar. While at least the primary almost fills its Roche lobe, the system is still detached, i.e. not yet a fully fledged W UMa-system. The behaviour of the Caii IRT line at 8662 A confirms that the secondary is the more active component, and that the chromospheric emission is not symmetrically distributed over the surfaces of either star.