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1 Introduction

The star HD 164492, of spectral type O7.5 III((f)) (Walborn 1973), is the central object of the multiple system ADS 10991. Undoubtedly it is responsible for the ionisation of the Trifid nebula (NGC 6514; M 20). According to ADS, the components of the system are: A - 8.0$^{\mathrm{m}}$, B - 10.6$^{\mathrm{m}}$ (AB 6''), C - 8.8$^{\mathrm{m}}$ (AC 11''), D - 10.5$^{\mathrm{m}}$ (CD 2''), E - 12.4$^{\mathrm{m}}$ (CE 6''), F - 13.8$^{\mathrm{m}}$ (AF 22'') and G - 13.2$^{\mathrm{m}}$ (CG 30''). Gahm et al. (1983) classified component B as A2 Ia, component C as B6 V and E as F3 V. The star was measured photoelectrically many times, however, the results of photometry differed. Therefore it was added to the New Catalogue of Suspected Variable Stars (Kukarkin et al. 1982) as NVS 10013. According to the spectroscopic data so far available, the radial velocities of components A and C are variable, too (see e.g. Abt & Biggs 1972).

Table 1 collects the published photometry of this star. Remarkable is the note by Borgman (1960) about the difference between his two measurements; it can be noted that another star for which Borgman gives similarly different data - HD 35921 - was later proved to be an eclipsing binary (LY Aur). In case of HD 164492, some differences in photometry may result from the diaphragm used: a different amount of the bright nebulosity as well as further components may enter the diaphragm. Unfortunately, the diaphragm diameters are rarely indicated by the authors, so it is difficult to judge the effects mentioned. The system was measured by HIPPARCOS (ESA 1997), however, the photometry has a large scatter due to the multiplicity (HIPPARCOS magnitudes are 7.5, 10.4 and 8.9 for components A, B and C, respectively).

Table 1: Previously published UBV data for HD 164492 (visual magnitudes similar to V system are also listed)

Source & $V$\space & $B{-}V$\...
 ...ponent A\\  & 8.671& & & components CD\\ \hline\noalign{\smallskip}\end{tabular}

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