  The K2.5 dwarf HD 40307 has been reported to host three super-Earths. The
system lacks massive planets and is therefore a potential candidate for having
additional low-mass planetary companions. We re-derive Doppler measurements
from public HARPS spectra of HD 40307 to confirm the significance of the
reported signals using independent data analysis methods. We also investigate
these measurements for additional low-amplitude signals. We used Bayesian
analysis of our radial velocities to estimate the probability densities of
different model parameters. We also estimated the relative probabilities of
models with differing numbers of Keplerian signals and verified their
significance using periodogram analyses. We investigated the relation of the
detected signals with the chromospheric emission of the star. As previously
reported for other objects, we found that radial velocity signals correlated
with the S-index are strongly wavelength dependent. We identify two additional
clear signals with periods of 34 and 51 days, both corresponding to planet
candidates with minimum masses a few times that of the Earth. An additional
sixth candidate is initially found at a period of 320 days. However, this
signal correlates strongly with the chromospheric emission from the star and is
also strongly wavelength dependent. When analysing the red half of the spectra
only, the five putative planetary signals are recovered together with a very
significant periodicity at about 200 days. This signal has a similar amplitude
as the other new signals reported in the current work and corresponds to a
planet candidate with M sin i = 7 Me (HD 40307 g). ...
