The tenth international GEOS symposium was held on 25–27 September 1986 in Las Fuentes de Alcoceber (Castellón, Spain). For the first time, the meeting took place in Spain and was organised by the Asociación Valenciana de Astronomía (AVA) with the collaboration of the Club Deportivo Las Fuentes, where the working sessions were held. A total of 46 participants attended, coming from Italy, France and Spain.
The symposium presentations were broadly divided into two categories. The first included theoretical contributions related to recent developments in stellar astrophysics. E. Poretti (Brera Observatory) presented a study on the interpretation of light curves of pulsating variables, while A. Buzzoni discussed stellar evolution near the asymptotic branch of the Hertzsprung–Russell diagram. F. Querci (Toulouse Observatory) analysed the structure of a complex stellar envelope, and J. Fabregat (University of Valencia) presented the possibility of detecting relativistic effects in binary systems.
Invited lectures also covered topics of general interest. V. Reglero (University of Valencia) reviewed recent discoveries concerning solar-type stellar variability and encouraged observations of such stars, which are accessible to amateur instrumentation. J. Torra (University of Barcelona) introduced the Hipparcos space project, aimed at producing a highly precise catalogue of stellar positions, magnitudes and colours.
The second group of contributions focused on observational work carried out within GEOS. E. Poretti reviewed the most important results obtained by the group, comparing them with professional data. J. Busquets and G. Bianciardi presented visual and photographic observations of the star V743 Lyr and of NSV 11944, respectively. P. Baruffetti reported on GEOS observational campaigns conducted in Loiano, while A. Figer described observations made at the European Southern Observatory (ESO) in Chile.
New observing programmes were proposed, including coordinated studies of semi-regular variables (F. Fumagalli and S. Ferrand) and a photoelectric photometry programme aimed at detecting gaseous envelopes around supergiant stars (F. Querci). A notable development was the creation of a photographic photometry section within GEOS, led by G. Bianciardi and F. Fumagalli.
One of the most significant proposals was presented by C. Gregory, who described the possibility of simultaneous visual and photoelectric observations using two telescopes at the Saint-Michel Observatory in France. This facility, made available to GEOS, would allow continuous monitoring through coordinated shifts of observers, potentially covering several months of uninterrupted observations.
In addition to the scientific programme, several social activities were organised, including a visit to Peñíscola, a traditional Valencian paella, and a friendly football match between Spain and the rest of the world.
The symposium concluded successfully, and during the final discussion it was suggested that the organisation of a future meeting could again be entrusted to the Asociación Valenciana de Astronomía. A new symposium in Las Fuentes was considered as a possibility, with the hope of broader participation.