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Context. The extrasolar planet HAT-P-8 b was thought to be one of the moreinflated transiting hot Jupiters. Aims. By using new and existing photometricdata, we computed precise estimates of the physical properties of the system.Methods. We present photometric observations comprising eleven light curvescovering six transit events, obtained using five medium-class telescopes andtelescope-defocussing technique. One transit was simultaneously obtainedthrough four optical filters, and two transits were followed contemporaneouslyfrom two observatories. We modelled these and seven published datasets usingthe jktebop code. The physical parameters of the system were obtained fromthese results and from published spectroscopic measurements. In addition, weinvestigated the theoretically-predicted variation of the apparent planetaryradius as a function of wavelength, covering the range 330-960 nm. Results. Wefind that HAT-P-8 b has a significantly lower radius (1.321 R_Jup) and mass(1.275 M_Jup) compared to previous estimates (1.50 R_Jup and 1.52 M_Juprespectively). We also detect a radius variation in the optical bands that,when compared with synthetic spectra of the planet, may indicate the presenceof a strong optical absorber, perhaps TiO and VO gases, near the terminator ofHAT-P-8 b. Conclusions. These new results imply that HAT-P-8 b is notsignificantly inflated, and that its position in the planetary mass-radiusdiagram is congruent with those of many other transiting extrasolar planets.

A lower radius and mass for the transiting extrasolar planet HAT-P-8b