Since 1950 when Oort published his paper on the structure of the cloud of comets it is believed that stars passing near this hypothetical cometary reservoir play an important role in the dynamical evolution of long period comets and injecting them into the observability region of the Solar System. The aim of this paper is to discuss two cases in which the data obtained from observations were used and stellar perturbations (of different intensity, strong case of C/2002 A3 LINEAR and weaker case of C/2013 F3 PANSTARRS) on cometary motion were detected. Using the best available data from the Gaia DR2 catalogue and some other sources we searched for close stellar passages near the Sun. Our study took into account that some of the stars are parts of multiple systems. Over 600 stars or systems that approached or will approach the Sun closer than 4.0 pc were found. Having the list of perturbers completed we studied their influence on a sample of 277 Oort spike comets that were observed since 1901 and discovered that two comets might have their orbits fundamentally changed due to a close stellar encounter. Our results show how much different the dynamical evolution of comets would have looked when their motion was considered only in the Galactic potential. Uncertainties both in stellar and cometary data were carefully taken into account. Our analysis indicates that the occurrence of stellar perturbations on cometary motions is very rare and the uncertainties of these effects are hard to estimate.

Context. The second Gaia data release (Gaia DR2) provided us with the precise five-parameter astrometry for 1.3 billion of sources. As stars passing close to the Solar System are thought to influence the dynamical history of long-period comets, we update and extend the list of stars that could potentially perturb the motion of these comets. Aims. We announce a publicly available database containing an up-to-date list of stars and stellar systems potentially perturbing the motion of long-period comets. We add new objects and revise previously published lists. Special emphasis is placed on stellar systems. A discussion of mass estimation is included. Methods. Using the astrometry, preferably from Gaia DR2, augmented with data from other sources, we calculate nominal spatial positions and velocities for each star. To filter studied objects on the basis of their nominal minimum heliocentric distances we numerically integrate the motion of stars under the Galactic potential and their mutual interactions. Results. We announce the updated list of stellar perturbers of cometary motion, including the masses of perturbers along with the publicly available database interface. These data are ready to be used with the observed long-period comets orbits to study an individual influence of a whole sample of perturbers, or specific stars, on a dynamical past or future of a specific comet. New potential perturbers were added; there are 138 more than in the previously published sources. Conclusions. We demonstrate that a new set of prospective perturbers is an important tool in studies of cometary dynamics. The use of our data changes the results of the past and future cometary motion analysis. We point out a puzzling object in our list, star ALS 9243. The Gaia DR2 astrometry suggests a very close encounter of this star with the Sun; however, its astrophysical parameters result in a completely different current distance of ALS 9243 and its high mass.