  The Rossiter-McLaughlin (RM) effect is a method that allows us to measure the
orbital obliquity of planets, which is an important constraint that has been
used to understand the formation and migration mechanisms of planets,
especially for hot Jupiters. In this paper, we present the RM observation of
the Neptune-sized long-period transiting planet HIP41378 d. Those observations
were obtained using the HARPS-N/TNG and ESPRESSO/ESO-VLT spectrographs over two
transit events in 2019 and 2022. The analysis of the data with both the
classical RM and the RM Revolutions methods allows us to confirm that the
orbital period of this planet is 278 days and that the planet is on a prograde
orbit with an obliquity of $\lambda$ = 57.1+26.4-17.9 degrees, a value which is
consistent between both methods. HIP41378 d is the longest period planet for
which the obliquity was measured so far. We do not detect transit timing
variations with a precision of 30 and 100 minutes for the 2019 and 2022
transits, respectively. This result also illustrates that the RM effect
provides a solution to follow-up from the ground the transit of small and
long-period planets such as those that will be detected by the forthcoming
ESA's PLATO mission.
