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We have investigated the relevant trend of the bolometric correction (BC) at the cool‐temperature regime of red giant stars and its possible dependence on stellar metallicity. Our analysis relies on a wide sample of optical–infrared spectroscopic observations, along the 3500 Å ⇒ 2.5 μm  wavelength range, for a grid of 92 red giant stars in five (three globular + two open) Galactic clusters, along the full metallicity range covered by the bulk of the stars,  −2.2 ≤[Fe/H]≤+0.4 .  Synthetic   BVR  C  I  C    JHK  photometry from the derived spectral energy distributions allowed us to obtain robust temperature ( T  eff ) estimates for each star, within  ±100 K  or less. According to the appropriate temperature estimate, blackbody extrapolation of the observed spectral energy distribution allowed us to assess the unsampled flux beyond the wavelength limits of our survey. For the bulk of our red giants, this fraction amounted to 15 per cent of the total bolometric luminosity, a figure that raises up to 30 per cent for the coolest targets  ( T  eff ≲ 3500 K) . Overall, we obtain stellar  M  bol  values with an internal accuracy of a few percentages. Even neglecting any correction for lost luminosity etc., we would be overestimating  M  bol  by ≲0.3 mag, in the worst cases. Making use of our new data base, we provide a set of fitting functions for the  V  and  K  BC versus  T  eff  and versus  ( B − V )  and  ( V − K )  broad‐band colours, valid over the interval  3300 ≤ T  eff ≤ 5000 K , especially suited for red giants.  The analysis of the  BC  V    and  BC  K    estimates along the wide range of metallicity spanned by our stellar sample shows no evident drift with [Fe/H]. Things may be different for the  B ‐band correction, where the blanketing effects are more and more severe. A drift of  Δ( B − V )  versus [Fe/H] is in fact clearly evident from our data, with metal‐poor stars displaying a ‘bluer’ ( B − V )  with respect to the metal‐rich sample, for fixed  T  eff .  Our empirical bolometric corrections are in good overall agreement with most of the existing theoretical and observational determinations, supporting the conclusion that (a)  BC  K    from the most recent studies are reliable within  ≲±0.1  over the whole colour/temperature range considered in this paper, and (b) the same conclusion apply to  BC  V    only for stars warmer than  ≃3800 K . At cooler temperatures the agreement is less general, and MARCS models are the only ones providing a satisfactory match to observations, in particular in the  BC  V    versus  ( B − V )  plane.

Bolometric correction and spectral energy distribution of cool stars in Galactic clusters ★