  Stellar metallicity -- as a probe of the metallicity of proto-planetary disks
-- is an important ingredient for giant planet formation, likely through its
effect on the timescales in which rocky/icy planet cores can form. Giant
planets have been found to be more frequent around metal-rich stars, in
agreement with predictions based on the core-accretion theory. In the
metal-poor regime, however, the frequency of planets, especially low-mass
planets, and how it depends on metallicity are still largely unknown. As part
of a planet search programme focused on metal-poor stars, we study the targets
from this survey that were observed with HARPS on more than 75 nights. The main
goals are to assess the presence of low-mass planets and provide a first
estimate of the frequency of Neptunes and super-Earths around metal-poor stars.
We perform a systematic search for planetary companions, both by analysing the
periodograms of the radial-velocities and by comparing, in a
statistically-meaningful way, models with an increasing number of Keplerians. A
first constraint on the frequency of planets in our metal-poor sample is
calculated considering the previous detection (in our sample) of a
Neptune-sized planet around HD175607 and one candidate planet (with an orbital
period of 68.42d and minimum mass $M_p \sin i = 11.14 \pm 2.47 M_{\oplus}$) for
HD87838, announced in the present study. This frequency is determined to be
close to 13% and is compared with results for solar-metallicity stars.
