  The magnetoelastic coupling in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ and in
Ca$_{1.5}$Sr$_{0.5}$RuO$_4$ has been studied combining high-resolution
dilatometer and diffraction techniques. Both compounds exhibit strong anomalies
in the thermal-expansion coefficient at zero and at high magnetic field as well
as an exceptionally large magnetostriction. All these structural effects, which
are strongest in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$, point to a redistribution of
electrons between the different $t_{2g}$ orbitals tuned by temperature and
magnetic field. The temperature and the field dependence of the
thermal-expansion anomalies in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ yield evidence for a
critical end-point lying close to the low-temperature metamagnetic transition;
however, the expected scaling relations are not well fulfilled.
