  In this paper we present ground-based high-resolution spectroscopy of 51
Pegasi using CRIRES at the Very Large Telescope. The system was observed for
3x5 hours at 2.3 {\mu}m at a spectral resolution of R = 100,000, targeting
potential signatures from carbon monoxide, water vapour and methane in the
planet's dayside spectrum. In the first 2x5 hours of data, we find a combined
signal from carbon monoxide and water in absorption at a formal 5.9{\sigma}
confidence level, indicating a non-inverted atmosphere. We derive a planet mass
of M_P = (0.46 +- 0.02) M_Jup and an orbital inclination i between 79.6 and
82.2 degrees, with the upper limit set by the non-detection of the planet
transit in previous photometric monitoring. However, there is no trace of the
signal in the final 5 hours of data. A statistical analysis indicates that the
signal from the first two nights is robust, but we find no compelling
explanation for its absence in the final night. The latter suffers from
stronger noise residuals and greater instrumental instability than the first
two nights, but these cannot fully account for the missing signal. It is
possible that the integrated dayside emission from 51 Peg b is instead strongly
affected by weather. However, more data are required before we can claim any
time variability in the planet's atmosphere.
