Cloud of Things (CoT) is a new paradigm that explores the synergy between Internet of Things (IoT) and cloud computing and has increasingly promoted the development of complex software-intensive systems in diverse and even critical application domains. In parallel, reference architectures have contributed as a means to capture the knowledge of how to develop, standardize, and evolve software-intensive systems and, as a consequence, have promoted quality issues in different domains. However, there is still a lack of investigation on reference architectures for CoT, which could be impacting the lack of standardization in CoT systems. Hence, the main contribution of this paper is to present an overview of reference architectures for CoT and, for this, we conducted a systematic mapping study (SMS) and found eight architectures proposed quite recently in different application domains. As a result, these architectures have advanced mainly in terms of standardization, but they have not still large dissemination and wider adoption by the software industry. Several open issues can be pointed out, including actions to promote the sustainability of these architectures.