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We analyse time-series observations from the BRITE-Constellation of the wellknown $\beta$ Cephei type star $\theta$ Ophiuchi. Seven previously knownfrequencies were confirmed and nineteen new frequency peaks were detected. Inparticular, high-order g modes, typical for the SPB (Slowly Pulsating B-typestar) pulsators, are uncovered. These low-frequency modes are also obtainedfrom the 7-year SMEI light curve. If g modes are associated with the primarycomponent of $\theta$ Oph, then our discovery allows, as in the case of otherhybrid pulsators, to infer more comprehensive information on the internalstructure. To this aim we perform in-depth seismic studies involvingsimultaneous fitting of mode frequencies, reproducing mode instability andadjusting the relative amplitude of the bolometric flux variations. To explainthe mode instability in the observed frequency range a significant increase ofthe mean opacity in the vicinity of the $Z$-bump is needed. Moreover,constraints on mass, overshooting from the convective core and rotation arederived. If the low-frequency modes come from the speckle B5 companion thentaking into account the effects of rotation is enough to explain thepulsational mode instability.

Seismic modelling of early B-type pulsators observed by BRITE: I. $\theta$ Ophiuchi