Eclipsed X-ray flares in binary stars: geometrical constraints on the flare's location and size

Aims. The observation of eclipses during X-ray flares taking place in active cool stars binaries allows us to calculate the position and size of the flares. This information cannot be derived by analyzing the decay of the flares, a frequently used approach in the literature that requires the assumption of a physical model. We make use of the eclipsing light curve to constrain the set of possible solutions, from the geometrical point of view, in two flares of Algol, and one flare in VW Cep. Methods. We make use of a technique developed with the system SV Cam (i similar to 90 degrees) and generalize it to binary systems with arbitrary inclination. The method simulates all possible geometrical situations that can produce the times of the four contacts of the eclipse. As an approximation we assume that the emitting region has a spherical shape that remains unchanged during the eclipse. We show, however, that this is a good approximation for the problem. Results. The solutions observed indicate that in two of the three cases the flare cannot be polar (vertical bar theta vertical bar < 55 degrees) and in a third one the flare can be placed either near the pole or at other latitudes. The emitting regions must have a small size (0.002-0.5 R*), but if interpreted as the apex of coronal loops, their length could actually be up to 3.1 R* for one of the Algol flares. These measurements imply a lower limit to the electron density in the emitting region between log ne (cm(-3)) 10.4 and 14.0, and a magnetic field between 70 and 3500 G. Similar results are found if the emitting region is assumed to be loop-shaped.