De 'rarus, -a, -um' adjectivo L&S, http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0059%3Aentry%3Drarus, wrote:b. Mostly poet. and in post-Aug. prose for the adv. raro,
seldom, rarely: “nec Iliacos coetus nisi rarus adibat,” Ov. M. 11, 766; cf.: “rarus, qui tam procul a portu recessisset, reperiebatur,” Quint. 12, prooem. § 3; so, “rarus fuit, qui, etc.,” id. 6, 2, 3: “antiquis scriptoribus rarus obtrectator,” Tac. A. 4, 33; Sall. H. Fragm. 3, 22, p. 233 Gerl.: “Caesar rarus egressu,” Tac. A. 15, 53; cf.: “leones rari in potu,” Plin. 8, 16, 18, § 46: “(calculus) rarus inventu,” id. 28, 15, 61, § 217; cf.: “helxine rara visu est,” id. 21, 16, 56, § 96: “Homerus alias circa picturas pigmentaque rarus,” i. e. rarely speaks of them, id. 33, 7, 38, § 115—
B. Of any thing found in small numbers or which seldom takes place, few, rare (cf. paucus): “in omni arte ... ut in ipsâ virtute, optimum quidque rarissimum,” Cic. Fin. 2, 25, 81; cf.: “rarum genus (amicorum) et quidem omnia praeclara rara,” id. Lael. 21, 79: “raris ac prope nullis portibus,” Caes. B. G. 3, 12 fin.; cf.: “etiamsi rarus ejus rei, nonnullus tamen usus,” Quint. 8, 6, 30: “rarus enim est animus ad ea defendenda,” Sall. H. 3, 61, 7 Dietsch: “Idem rarum est, non sine usu tamen,” Quint. 5, 11, 42: “rari domos, plurimi amicorum tecta ... petivere,” Tac. H. 1, 79 fin.: “Oceanus raris ab orbe nostro navibus aditur,” id. G. 2: “aliquod solitarium aut rarum,” Cic. Inv. 1, 44, 83: “ut anteponantur rara vulgaribus,” id. Top. 18, 69: “litterae,” Liv. 6, 1; cf. id. 7, 3: “rara hostium apparebant arma,” id. 2, 50: “lites,” Quint. 7, 1, 43: “infelicitas,” id. 11, 2, 49: “quae (littera) est apud nos rarissima in clausulis,” id. 12, 10, 31: “quod est magis rarum,” id. 9, 2, 73: “ex maxime raro genere hominum,” Cic. Lael. 17, 64; cf. Quint. 7, 3, 25: “raris vocibus hisco,” Verg. A. 3, 314: “rara per ignotos errent animalia montes,” id. E. 6, 40: “audiet pugnas vitio parentum Rara juventus,” Hor. C. 1, 2, 24. —Rarum est, with ut: “rarum est, ut, etc.,” Quint. 3, 10, 3; 6, 3, 38; 10, 7, 24: “rarum dictu, esse aliquid, cui prosit neglegentia,” Plin. 18, 16, 39, § 140.