Two serendipitous low-mass LMC clusters discovered with HST

We present V and I photometry of two open clusters in the LMC down to V similar to 26. The clusters were imaged with the Wide Field and Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope (HST), as part of the Medium Deep Survey Key Project. Both are low-luminosity (M-v similar to-3.5), low-mass (M similar to 10(3) M-circle dot) systems. The chance discovery of these two clusters in two parallel WFPC2 fields suggests a significant incompleteness in the LMC cluster census near the bar. One of the clusters is roughly elliptical and compact, with a steep light profile, a central surface brightness mu(v)(0)similar to 20.2 mag arcsec(-2), a half-light radius r(hl)similar to 0.9 pc (total visual major diameter D similar to 3 pc) and an estimated mass M similar to 1500 M-circle dot. From the colour-magnitude diagram and isochrone fits we estimate its age as tau similar to(2-5) x 10(8) yr. Its mass function has a fitted slope of Gamma = Delta log phi (M)Delta logM = -1.8 +/- 0.7 in the range probed (0.9 less than or similar to M/M-circle dot less than or similar to 4.5). The other cluster is more irregular and sparse, having shallower density and surface brightness profiles. We obtain Gamma= -1.2+/-0.4, and estimate its mass as M similar to 400 M-circle dot. A derived upper limit for its age is tau less than or similar to 5 x 10(8) yr. Both clusters have mass functions with slopes similar to that of R136, a massive LMC cluster, for which HST results indicate Gamma similar to-1.2. They also seem to be relaxed in their cores and well contained in their tidal radii.