Proposal #86

Target Name: V777 Sgr

Type: EA/GS (β Persei-type (Algol) eclipsing binary with one or more giant or supergiant components)

R.A., Dec: 17 46 25.39, -26 12 01.1

Magnitude: 8.59 - 8.67 V, 10.35 – 10.68 in B
From our 2016 measurements eclipse depth was found to be ~0.08 in V and ~0.33 in B.

Background: The long period (~936 d) eclipsing binary V777 Sgr will be undergoing an eclipse from October to December this year. Time of mid eclipse is predicted to be JD 2458426.491 or 3 November, 2018. During the ingress phase, between about 4th to 9th October, V777 Sgr will be well positioned for early evening observations. It will be too low in the sky during the egress phase.

During the last eclipse in 2016 BSM-South and/or BSM-Berry observed the V777 Sgr each clear night for about 250 days starting a few days prior to first contact, through until well beyond fourth contact. These observations, along with those of several Variable Stars South observers allowed an improved estimate of the orbital period. However, for the 2016 eclipse the ingress and egress phases were poorly sampled. Thus we have no useful measurements of the relative diameter of the hot blue component being eclipsed.
We have checked the literature and southern sky survey databases such as ASAS and ASAS-SN but have not found suitable photometry during the ingress or egress phases for any previous eclipse.

Aim: During the upcoming 2018 eclipse we aim to obtain well sampled B and V light curves of the ingress phase including a week or more prior to first contact and a week or more after second contact. This will require observations from as large a spread of longitude as possible. We have observers in New Zealand, eastern Australia and South Africa. The inclusion of BSM-South (Victoria, Australia) and BSM-Berry (Western Australia) would provide valuable additional observations.

We request that BSM-South and BSM-Berry observe V777 Sgr each clear night between September 15th (or as soon as possible) through until October 25th. A set of 5 images in B and 5 images in V should be recorded at the end of astronomical twilight. These should be repeated each hour until V777 Sgr reaches an altitude of 30 degrees.

Initially V exposures of 10 sec and B exposures of 30 sec are requested, but these will be adjusted as necessary to achieve high SNR.