Why hot horizontal branch stars can appear redder than red giants

In this paper we report on a curious feature in the V, (U - B) color-magnitude diagrams of globular clusters. In our database, we find that a considerable fraction of blue horizontal branch stars, hotter than the instability strip and cooler than the Grundahl et al. (1999) jump (i.e., 6000 less than or similar to T-eff(K) less than or similar to 10 000), have (U - B) colors redder than their red giant progenitors. This red incursion is not expected on theoretical grounds, as horizontal branch stars (whose convective regions are less extended than in red giant structures) should not appear cooler than a red giant. Analyzing data from different telescopes we show that: 1) the horizontal branch red incursion is strongly dependent on the shape of the adopted U filter and to a lesser extent, on the B filter; 2) the photometry done with U filters that do not encompass the Balmer jump shows the blue horizontal branch red incursion; 3) the occurrence of this feature is also due to the peculiar dependence of the U and B magnitudes on star effective temperature, gravity, and metallicity; 4) theoretical tracks can reproduce the observed horizontal branch morphology, provided that the appropriate (i.e. exactly responding to the filters effectively used in the observations) transmission curve efficiencies are used for deriving color-T-eff transformations; 5) the red incursion extent depends on metallicity.