Four new binary minor planets: (854) Frostia, (1089) Tama, (1313) Berna, (4492) Debussy

Aims. We present evidence that four minor planets of the main belt are binary systems. Methods. These discoveries are based on CCD photometric measurements made by many observers coordinated in a network of observatories. Results. Orbital and physical properties are derived from a total of 134 partial light curves involving 26 stations. (854) Frostia, (1089) Tama, (1313) Berna, and (4492) Debussy show mutual eclipses features on their light curves.

In all cases, rotation and revolution are synchronous. Synodic periods are 37.728, 16.444, 25.464 and 26.606 h respectively. From a simple model, we have derived their bulk densities as follows: 0.89 +/- 0.14, 2.52 +/- 0.30, 1.22 +/- 0.15 and 0.91 +/- 0.10 g cm(-3) respectively. Uncertainties in the bulk densities, arising from scattering and shadow effects are not taken into account.

These could increase the density estimates by a factor up to 1.6. Our method of determining bulk density is completely independent of their mass and their diameter estimates. The low rotational periods and the low bulk densities clearly imply a collisional process to explain this kind of binary asteroid.

Based on our database of a few thousand light curves of minor planets, the population of similar-sized objects in the main belt is estimated to 6 +/- 3 percent in the 10-50 km diameter class.