Severan denarii CAN be that lightweight, so the weight doesn't condemn the coin by itself. Here's my example of the coin: Julia Domna, AD 193-211 Roman AR denarius; 3.24 gm, 19.8 mm Rome mint, AD 207 Obv: IVLIA AVGVSTA, bare-headed and draped bust, r. Rev: FECVNDITAS, Terra reclining l. under tree, left arm on basket of fruits, right hand set on globe, spangled with stars; in background, four children advancing r., representing the four seasons. Refs: RIC 549; BMCRE 21; Sear 6579
RI Julia Domna 193-195 CE AR Den Vesta Palladium RI AR Den Julia Domna 193-217 CE VENER VICT Venus RIC IVa p176 RI AR Den Julia Domna 200 CE Felicitas Isis Horus RIC 577