The search for roAp stars: null results and new candidates from
  Stroemgren-Crawford photometry
  The rapidly oscillating Ap (roAp) stars exhibit pulsational photometric
and/or radial velocity variations on time scales of several minutes, which are
essential to test current pulsation models as well as our assumptions of
atmospheric structure characteristics. In addition, their chemical peculiarity
makes them very interesting for probing stellar formation and evolution in the
presence of a global magnetic field. To date, a limited number of only 61 roAp
stars are known to show photometric variability. On the other hand, a
literature survey yields 619 unique stars that have unsuccessfully been
searched for variability of this kind. Stroemgren-Crawford uvbybeta photometry
of stars from both subgroups were used to investigate whether there is a
selection bias of the investigated stars. We also present new photometric
measurements (202 hours in 59 different nights) of 55 roAp candidates.We did
not detect any new roAp star. Although our detection limits are comparable to
other surveys, we also did not find pulsations in the known roAp star HD 12098,
which may be a consequence of temporal amplitude changes. On the other hand, we
do find some evidence for photometric variability of beta CrB at its
spectroscopically derived pulsation period. From the uvbybeta photometry we
conclude that the blue border of the roAp instability strip appears
observationally well defined, whereas the red border is rather poorly known and
studied. Within these boundaries, a total of 4646 candidates were identified
which appear worthwhile to be investigated for short-term pulsational
variability.
