The fourth annual GEOS symposium was held in Marly‑le‑Roi on 2–4 February 1979, with most participants present from Friday morning, allowing a more balanced three‑day programme. Talks were shorter and better prepared than in previous years, and simultaneous translation by C. Romoli, M. Penna and E. Poretti was described as “entirely satisfactory.” The meeting focused on observational results, theoretical presentations, and programme organisation.
L. Maurin opened with an analysis of suspected variability in 2 Per, a comparison star for 1 Per. Observations from the 1975 and 1978 GEOS camps showed recurring nightly variations of 0.06–0.10 mag, suggesting that “the variability of the comparison star B… seems the most likely explanation.” A possible 18.4–18.6‑day period was proposed, though confirmation remained pending.
A. Figer presented extensive observations of BL Cam, a very rapid RRc star. Early data contradicted the published 56.3‑minute period, but later coordinated observations allowed refinement to 0.0390798 d, slightly different from the original value. The star was considered ideal for confirmation during summer camps.
Other observational reports included UY Ari (A. Figer), where a combined analysis of 50 minima yielded a period of 0.3222 d, now considered secure for IBVS publication. Additional contributions covered RZ Cep, BW Vul, and several semi‑regular variables.
Theoretical sessions addressed main‑sequence variables (J. Lecacheux), night vision (Y. Dargery), and stellar pulsation, including a discussion of atmospheric structure as the driver of pulsation. S. Wabnitz reviewed Beat Cepheids and the need for numerous visual estimates.
Campaign planning included A. Buzzoni’s 1979 programme on bright eclipsing binaries, targeting apsidal motion and orbital solutions. A. Buzzoni also presented a methodological review of eclipsing‑binary photometry, noting that giving minima times “to three decimals is useless unless at least 60–100 estimates per hour are obtained.”
Further results included orbital modelling of AY Peg and NN Cep by A. Buzzoni and A. Gaspani, with NN Cep classified as a detached system. Technical presentations covered photoelectric photometry, hydrogen hypersensitisation of photographic plates and J.P. Clovin’s “Variosimulapex” training device.
A major debate clarified the distinction between “under‑studied stars” and well‑known eclipsing binaries. A limited list of eclipsing stars for routine minima monitoring would be established to reduce dispersion. Practical workshops tested visual estimation accuracy and graphical minima determination.
The symposium concluded that the annual meeting format was now “well‑established,” but participation from French observers had declined. As a result, the organiser announced the intention not to reserve Marly for 1980 and to cancel the 1980 meeting.