  (Abridged) The detection of forming planets in disks around young stars
remains elusive, and state-of-the-art observational techniques provide somewhat
ambiguous results. It has been reported that the pre-transitional T Tauri star
LkCa 15 could host three planets; candidate planet b is in the process of
formation, as inferred from its H$\alpha$ emission. However, a more recent work
casts doubts on the planetary nature of the previous detections. We have
observed LkCa 15 with ISIS/WHT. The spectrograph's slit was oriented towards
the last reported position of LkCa 15 b (parallel direction) and 90degr from
that (perpendicular). The photocenter and full width half maximum (FWHM) of the
Gaussians fitting the spatial distribution at H$\alpha$ and the adjacent
continuum were measured. A well-known binary (GU CMa) was used as a calibrator
to test the spectro-astrometric performance of ISIS/WHT, recovering consistent
photocenter and FWHM signals. However, the photocenter shift predicted for LkCa
15 b is not detected, but the FWHM in H$\alpha$ is broader than in the
continuum for both slit positions. Our simulations show that the photocenter
and FWHM observations cannot be explained simultaneously by an accreting
planet. In turn, both spectro-astrometric observations are naturally reproduced
from a roughly symmetric Halpha emitting region centered on the star and extent
comparable to the orbit originally attributed to the planet at several au. The
extended H$\alpha$ emission around LkCa 15 could be related to a variable disk
wind, but additional multi-epoch data and detailed modeling are necessary to
understand its physical nature. Spectro-astrometry in H$\alpha$ is able to test
the presence of accreting planets and can be used as a complementary technique
to survey planet formation in circumstellar disks.
