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Astron. Astrophys. 350, 566-570 (1999)

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2. Observations and reductions

OT Gem has been observed at Hvar Observatory as a part of the international campaign on photometry of bright Be stars (Pavlovski et al. 1997) from 1982 to 1985. Observations were resumed in 1996 to check on the long-term variations. All Hvar measurements were carefully reduced and transformed to the Johnson UBV system with the help of the reduction program HEC22 - see Harmanec et al. (1994) and Harmanec & Horn (1998). The journal of Hvar observations is presented in Table 1. The long-term light changes, already reported by Pavlovski et al. (1997), are clearly visible in later years. The brightness increase observed in the last seasons is in accordance with the result of Ferro et al. (1998). In order to give a more detailed description of the light and colour changes, we collected all available published photometric measurements of OT Gem. Basic information about the data sets used to study long and medium term variations is collected in Table 2.


[TABLE]

Table 1. Journal of Hvar UBV observations of OT Gem.
Comment:
All observations were made by 0.65 m Cassegrain reflector at Hvar Observatory with uncooled EMI 6256S photomultiplier. UBV magnitudes for the comparison star 1 CMi and for the check star HR 2858 are from Harmanec et al. (1994).



[TABLE]

Table 2. Published photoelectric observations of OT Gem.
Sources:
1. Poretti (1982); 2. Ferro et al. (1998); 3. Dachs et al. (1988); 4. Perryman et al. (1997); 5. Schuster & Guichard (1984)


In order to transform the 13-colour photometry of Schuster & Guichard (1984) to the standard UBV system, we selected from Table 7 of Johnson & Mitchell (1975) those stars which were used by Harmanec (1998b) to derive the [FORMULA] to UBV transformation. They have accurate UBV magnitudes derived from a huge amount of photoelectric observations secured at Hvar and Skalnaté Pleso Observatories (Harmanec et al. 1994) and were checked for constancy by comparing with the Hipparcos photometry. We arrived at the following transformation formulas:

[EQUATION]

To obtain the V magnitudes of OT Gem published by Ferro et al. (1998), we proceeded as follows. First, we derived the mean V magnitude of their comparison star HR 2780 from the transformed Hipparcos [FORMULA] magnitude. This value, 6.[FORMULA] 451, was then added to the magnitude differences OT Gem- HR 2780 from Table 1 of Ferro et al. (1998).

Hipparcos [FORMULA] magnitudes published by Perryman et al. (1997) were transformed to Johnson V and B magnitudes with the help of the transformation formulas derived by Harmanec (1998b).

Transformed Hipparcos and 13-colour photometry, La Luz and Hvar measurements are collected in Table 3. 1

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
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