The fifth annual GEOS symposium took place in Marly‑le‑Roi on 25–27 April 1980, gathering more than 40 observers from France, Italy, Belgium and Spain. The meeting was considered highly successful thanks to renewed participation and high‑quality presentations, including several by invited professional astronomers. Simultaneous translation was again provided and described as “entirely satisfactory.”
Observational results opened the programme. A. Gaspani presented studies of Delta Orionis and Eta Orionis, including revised ephemerides, apsidal‑motion analysis, and new light curves—the first complete one for Eta Ori since 1916. A. Figer reported on VZ Draconis, confirming it as an RRc star with a refined period of 0.32103 d, enabling linkage to 1961 observations. A. Figer also summarised new results on 1 Persei, including revised eclipse durations and amplitudes, and evidence that earlier published minima by Kurtz likely contained transcription errors.
Other observational contributions included BL Cam, UY Ari, V566 Oph and WY Gem. Theoretical sessions covered main‑sequence variables, night vision, stellar pulsation, Beat Cepheids, and detailed astrophysical reviews of Ap, Be and cool stars by professional researchers. M. Petit presented extensive work on Cepheid period variations and photometric criteria distinguishing population types.
Several methodological presentations addressed new techniques for determining minima, orbital‑element computation on programmable calculators, and improved approaches to eclipsing‑binary photometry. A planned debate on GEOS scientific orientation was postponed due to lack of time.
Campaign proposals included a major coordinated effort on CY Aqr during a photoelectric mission at the Chiran Observatory in August 1980, described as “an exceptional opportunity” for simultaneous visual and photoelectric observations. Additional proposals concerned suspected bright variables and a newly suspected RR‑type star in Hercules.
The symposium also discussed the 1980 summer camp, a proposal for international language‑exchange visits among GEOS members, and two “slide evenings” featuring astronomical films and images. The meeting concluded that Marly 80 was “probably the best” of the five annual symposia held so far.