Hydrogen (red circles) and Ca (blue squares) abundance versus T_eff. The red dotted line is an empirical upper limit to H abundances, drawn through those objects with the highest abundances. The continuous line is shifted downward by 5.7 dex, the solar H/Ca ratio. Stars with a solar or higher H/Ca would be found below this line, which is, however, below the visibility limit of Ca for all but the coolest stars.