HD-V. Constraints on the speed v (left) and vector-to-tensor fraction r² (middle) for vector modes mixing with luminal GWs (HD) in the SGWB derived from NANOGrav (Agazie et al. 2023b) (black dots) and CPTA (Xu et al. 2023) (green crosses) correlation data. Right panel: best-fit correlation samples (red curve with 1σ bands) corresponding to the left and middle panels. The HD curve (gray dashed) is shown for reference.