Resumen:
The study of transneptunian objects (TNOs) and Centaurs, because to their low brightness, is not simple. However, great contributions to the study and characterization of these objects have been obtained from the application of the stellar occultation technique. The stellar occultation technique is independent of the brightness of the occulting body and allows, among others, the determination of shapes/sizes with kilometric accuracy as well as the detection of a possible atmosphere. The prediction of these events depend on stellar positions and on the ephemerides of Solar System bodies. Stellar positions, thanks to Gaia, are available with unprecedented accuracy ($< 1\,\,mas$). On the other hand, the ephemerides of TNOs/Centaurs still need to be refined to provide accurate predictions. In this work, we determined the positions of 41 TNOs and 7 Centaurs as a contribution to the improvement of their orbits and thus to the accuracy in the prediction of stellar occultations by them. The astrometric results shown here were obtained with the help of the astrometry from the Gaia mission (Gaia DR2 catalogue), in two ways: through direct imaging taken as obtained at ESO (La Silla) with the WFI camera installed at the $2.2\,\,m$ telescope during four observation runs (2012B, 2013A, 2015B, 2016A) and from a position of the Centaur (10199) Chariklo as obtained through a stellar occultation occurred on 24th August 2017. Accuracies in positions obtained from astrometry through direct imaging are better than $20\,\,mas$ in both coordinates. The residuals with respect to NIMA ephemeris are typically less than $20\,\,mas$ (absolute value). The accuracy in position obtained from the stellar occultation was $\sigma_\alpha\cos\delta=2\,\,mas$ and $\sigma_\delta=4\,\,mas$. The difference with respect to the lastest version of NIMA ephemeris (v15), at the time of this writing, was $\Delta\alpha\cos\delta=-3\,\,mas$ and $\Delta\delta=-4\,\,mas$, in the sense observation minus NIMA. \\
