ROSAT PSPC observation of the X-ray faint early-type galaxy NGC 5866.

We present the results of the analysis of the ROSAT PSPC pointed observation of the S0 galaxy NGC 5866. Previous Einstein observations had revealed that this galaxy has a low X-ray to optical ratio L_X_/L_B_. Theoretical models of the X-ray emission of early-type galaxies had suggested that in objects of this kind the emission is not dominated by the presence of a hot diffuse gas, but should originate in stellar sources. We discuss the evidence in favor of this hypothesis following from the results of the analysis of the ROSAT PSPC data. The X-ray spectrum shows properties similar to those of the other two low L_X_/L_B_ early-type galaxies observed by ROSAT so far, including the presence of very soft emission. So, these galaxies can be recognized as a group with homogeneous properties, and a more exhaustive picture of the nature of the X-ray emission across the L_X_-L_B_ plane of early-type galaxies can be outlined. We also discuss the importance of rotation in the X-ray emission of S0 galaxies, and suggest that it could explain why on average S0s are less X-ray luminous than ellipticals.