VLBI observations of two single dMe stars: spatial resolution and astrometry

We report on 3.6 cm VLA and VLBA observations of YZ CMi and AD Leo, two nearby dMe stars. We resolve YZ CMi and can fit a circular symmetrical gaussian component of FWHP of 0.98 ±0.2 mas, corresponding to an extent of the corona above the photosphere of 1.77 × 1010 ± 8.8 × 109 cm or 0.7 ±0.3 R? (R? refers to the photospheric radius). We obtain an estimate of the brightness temperature of 7.3 × 107 K, which is consistent with that expected from gyrosynchrotron emission. For AD Leo the emitting region is unresolved. We therefore set a conservative upper limit to its diameter of 1.8 times the photosphere diameter, which leads to an extent of the corona above the photosphere of < 2.8 × 1010cm or < 0.8 R?. We compare the radio emitting dMe stars with measured sizes with the Sun and conclude that these active stars have much more extended coronal radio emission than the Sun. The VLBA position of YZ CMi has been found to differ by 32 mas from the positions calculated from the Hipparcos catalogue. The discrepancy is caused by large errors in the listed proper motion. An improved value is given.