This paper presents ORB-LINE-SLAM, a real-time hybrid point-line and only-line based visual SLAM system for stereo cameras which can operate in standard CPUs. This work is based on the ORB-SLAM3 open-source library, which is here expanded and adapted to further cope with line features. The main contribution of this paper concerns the introduction of an adaptation scheme, based on an experimentally tuned adapting factor, which aims to achieve a more efficient fusion of point and line features during the SLAM process. Furthermore, to the best of our knowledge, this work constitutes the first open-source visual SLAM system that has the ability to perform effectively by using exclusively line features. This paper also performs a systematic comparison of different error functions used in the bundle adjustment process, to deduce which ones are the more efficient for the visual SLAM problem. The experimental results obtained indicate that the proposed point-line method significantly improves the accuracy of ORB-SLAM3 in challenging conditions. Furthermore, the only-line method implemented in this work, despite its simplified nature, also manages to work in a standalone mode and to successfully complete even difficult sequences with remarkable precision.