  Astrometric monitoring of the Sirius binary system over the past century has
yielded several predictions for an unseen third system component, the most
recent one suggesting a \leq50 MJup object in a ~6.3-year orbit around Sirius
A. Here we present two epochs of high-contrast imaging observations performed
with Subaru IRCS and AO188 in the 4.05 \mum narrow-band Br alpha filter. These
data surpass previous observations by an order of magnitude in detectable
companion mass, allowing us to probe the relevant separation range down to the
planetary mass regime (6-12 M_Jup at 1", 2-4 M_Jup at 2", and 1.6 M_Jup beyond
4"). We complement these data with one epoch of M-band observations from MMT/AO
Clio, which reach comparable performance. No dataset reveals any companion
candidates above the 5-sigma level, allowing us to refute the existence of
Sirius C as suggested by the previous astrometric analysis. Furthermore, our Br
alpha photometry of Sirius B confirms the lack of an infrared excess beyond the
white dwarf's blackbody spectrum.
