Chill the Cava, gin and vodka in the fridge for at least 1–2 hours.
In a jug (at least 1.5 litres), combine the orange juice, gin and vodka. Add the caster sugar, if using, and stir until it has completely dissolved.
Just before serving, carefully open the very cold Cava and gently pour it into the jug. Stir lightly to preserve the bubbles.
Fill tumblers with plenty of ice, divide the cocktail between them and garnish each with an orange slice. Serve immediately.