  We have observed 7 new transits of the `hot Jupiter' WASP-5b using a 61 cm
telescope located in New Zealand, in order to search for transit timing
variations (TTVs) which can be induced by additional bodies existing in the
system. When combined with other available photometric and radial velocity (RV)
data, we find that its transit timings do not match a linear ephemeris; the
best fit \chi^2 values is 32.2 with 9 degrees of freedom which corresponds to a
confidence level of 99.982 % or 3.7 \sigma. This result indicates that excess
variations of transit timings has been observed, due either to unknown
systematic effects or possibly to real TTVs. The TTV amplitude is as large as
50 s, and if this is real, it cannot be explained by other effects than that
due to an additional body or bodies. From the RV data, we put an upper limit on
the RV amplitude caused by the possible secondary body (planet) as 21 m s^{-1},
which corresponds to its mass of 22-70 M_{Earth} over the orbital period ratio
of the two planets from 0.2 to 5.0. From the TTVs data, using the numerical
simulations, we place more stringent limits down to 2 M_{Earth} near 1:2 and
2:1 mean motion resonances (MMRs) with WASP-5b at the 3 \sigma level, assuming
that the two planets are co-planer. We also put an upper limit on excess of
Trojan mass as 43 M_{Earth} (3 \sigma) using both RV and photometric data. We
also find that if the possible secondary planet has non- or a small
eccentricity, its orbit would likely be near low-order MMRs. Further follow-up
photometric and spectroscopic observations will be required to confirm the
reality of the TTV signal, and results such as these will provide important
information for the migration mechanisms of planetary systems.
