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The obscured OH/IR star IRAS05298-6957 in the LMC was recently noticed to bemember of the small double cluster HS 327 that also contains a carbon star (vanLoon et al., 1998, A&A 329, 169). Hence they are coeval and have (nearly) thesame progenitor mass, which can only be understood if Hot Bottom Burning (HBB)has prevented IRAS05298-6957 from being a carbon star. We present extensivevisual and near-IR photometric data for >10^4 stars in and around HS 327, andspectroscopic data for some of the brightest AGB stars amongst these.Colour-magnitude diagrams are used to estimate the age for the cluster and itsmembers, and luminosities are derived for the stars for which spectra have beenobtained. The age for IRAS05298-6957 and the carbon star is estimated to beabout 200 Myr. This corresponds to a Main-Sequence progenitor mass of about 4.0M_sun --- the first direct measurement of the lower mass threshold for HBB.This agrees with stellar evolution models that, however, fail to reproduce thelow luminosity of the carbon star.Comment: 11 pages (better quality Figs 1b & 9 on request from  jacco@ast.cam.ac.uk). Accepted for publication in Astronomy and Astrophysics  Main Journa

The peculiar cluster HS 327 in the Large Magellanic Cloud: Can OH/IR stars and carbon stars be twins?