  The structure and magnetic interactions of the triplon Bose-Einstein
condensation candidates Ba$_3$Cr$_2$O$_8$ and Sr$_3$Cr$_2$O$_8$ have been
studied thoroughly in the literature, but little is known about a possible
triplon condensation in the corresponding solid solution
Ba$_{3-x}$Sr$_x$Cr$_2$O$_8$. We have prepared various members of this solid
solution and systematically examined their magnetic properties in high magnetic
fields up to 60 T and at low temperatures down to 340 mK, by means of pulsed
field and cantilever magnetometry. From these experiments for
$x\in\{3,2.9,2.8,2.7,2.6,2.5\}$, we find that the critical fields of
Ba$_{3-x}$Sr$_x$Cr$_2$O$_8$ decrease monotonically as a function of the Sr
content $x$. This change is in good agreement with the earlier reported
variation of the magnetic interactions in these compounds.
