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(Abridged) Transition disks are recognized by the absence of emission ofsmall dust grains inside a radius of up to several 10s of AUs. Due to the lackof angular resolution and sensitivity, the gas content of such dust holes hasnot yet been determined, but is of importance to constrain the mechanismleading to the dust holes. Transition disks are thought to currently undergothe process of dispersal, setting an end to the giant planet formation process.  We present new high-resolution observations with the Atacama LargeMillimeter/ submillimeter Array (ALMA) of gas lines towards the transition diskOph IRS 48 previously shown to host a large dust trap. ALMA has detected the$J=6-5$ line of $^{12}$CO and C$^{17}$O around 690 GHz (434 $\mu$m) at aresolution of $\sim$0.25$''$ corresponding to $\sim$30 AU (FWHM). The observedgas lines are used to set constraints on the gas surface density profile.  New models of the physical-chemical structure of gas and dust in Oph IRS 48are developed to reproduce the CO line emission together with the spectralenergy distribution (SED) and the VLT-VISIR 18.7 $\mu$m dust continuum images.Integrated intensity cuts and the total spectrum from models having differenttrial gas surface density profiles are compared to observations. Using thederived surface density profiles, predictions for other CO isotopologues aremade, which can be tested by future ALMA observations of the object.  The derived gas surface density profile points to the clearing of the cavityby one or more massive planet/companion rather than just photoevaporation orgrain-growth.

Gas structure inside dust cavities of transition disks: Oph IRS 48 observed by ALMA